Epidemiology of Substance Use among Forced Migrants: A Global Systematic Review

Introduction Forced migration is occurring at unprecedented levels. Forced migrants may be at risk for substance use for reasons including coping with traumatic experiences, co-morbid mental health disorders, acculturation challenges and social and economic inequality. This paper aimed to systematically review the literature examining substance use among forced migrants, and identify priority areas for intervention and future research. Methods Seven medical, allied health and social science databases were searched from inception to September 2015 in accordance with PRISMA guidelines to identify original peer-reviewed articles describing any findings relating to alcohol and/or illicit drug use among refugees, internally displaced people (IDPs), asylum seekers, people displaced by disasters and deportees. A descriptive synthesis of evidence from quantitative studies was conducted, focusing primarily on studies which used validated measures of substance use. Synthesis of evidence from qualitative studies focused on identifying prominent themes relating to the contexts and consequences of substance use. Critical Appraisal Skills Programme (CASP) checklists were used to assess methodological quality of included studies. Results Forty-four quantitative (82% cross-sectional), 16 qualitative and three mixed-methods studies were included. Ten studies were rated as high methodological quality (16%), 39 as moderate quality (62%) and 14 as low quality (22%). The majority of research was conducted among refugees, IDPs and asylum seekers (n = 55, 87%), predominantly in high-income settings. The highest-quality prevalence estimates of hazardous/harmful alcohol use ranged from 17%-36% in camp settings and 4%-7% in community settings. Few studies collected validated measures of illicit drug use. Seven studies compared substance use among forced migrants to other migrant or native-born samples. Among eight studies which conducted multivariable analysis, male sex, trauma exposure and symptoms of mental illness were commonly identified correlates of substance use. Conclusion Our understanding of substance use among forced migrants remains limited, particularly regarding persons displaced due to disasters, development and deportation. Despite a growing body of work among refugee-background populations, few studies include refugees in low and middle-income countries, where over 80% of the global refugee population resides. Findings suggest a need to integrate substance use prevention and treatment into services offered to forced migrants, particularly in camp settings. Efforts to develop and evaluate interventions to reduce substance use and related harms are needed.


Drivers and trends in forced migration
The International Organisation for Migration (IOM) defines forced migration as "a migratory movement in which an element of coercion exists, including threats to life and livelihood, whether arising from natural or man-made causes" [1]. Three main causes of forced migration are commonly considered: conflict, disaster, and development [2].
Although the number of active conflicts globally decreased from 63 in 2008 to 42 in 2014 [3], the impacts on civilian populations are intensifying, with the number of people displaced due to conflict reaching unprecedented levels. At the end of 2014, 59.5 million people were displaced as a result of violence and persecution, an increase of 8.3 million from the previous year, among the highest annual increases ever recorded [4]. This figure comprises 19.5 million refugees (people who meet the definition provided by the 1951 United Nations (UN) Convention and its 1967 Protocol), 38.2 million internally displaced persons (IDPs; persons who essentially meet the definition of a refugee but who have not crossed an internationally recognised state border [1]), and 1.8 million asylum seekers whose claims await assessment [4]. The ongoing conflict in the Syrian Arab Republic, now in its fourth year, contributed significantly to global displacement in 2014, accounting for almost 3.9 million refugees and 7.6 million IDPs, and overtaking Afghanistan as the largest refugee source country [4].
There are three traditional 'durable solutions' for refugees: voluntary repatriation, local integration and resettlement. Ongoing political instability and general insecurity contribute to low levels of voluntary repatriation. Resettlement demand outweighs opportunity; in 2014, just 105,000 refugees were resettled in 26 countries, with the United States (US), Canada and Australia granting permanent residence to the largest numbers [4]. As a result, most refugees remain in countries of first asylum. While many reside in formal camps, an increasing proportion live informally in urban settings [4,5].
A second key driver of forced migration globally is environmental change [6]. Weatherrelated natural disasters such as cyclones or floods play a role in sudden large-scale displacement due to the destruction of homes and livelihoods. Environmental change also contributes to slow-onset migration, for example, due to food insecurity and famine brought on by drought, and the impacts of rising sea levels on low-lying areas and small island states [7]. Although numbers vary drastically by year, there has been a general increasing trend in disasters and disaster-induced displacement, with 315 disaster events and 22 million people displaced due to disasters in 2013 [8,9]. With the effects of global warming predicted to increase [6], the number of environmental migrants is expected to reach 200 million by 2050 [10]. Following initial evacuation at the time of a disaster, displacees may return to their original homes, or relocate temporarily or permanently. Most disaster-induced displacees who cannot return home remain within their country of origin, and can be considered IDPs.
Development is a third key driver of forced migration. Development projects, such as infrastructure projects or the extraction of natural resources, promise significant economic development, however, these commonly require the acquisition of land which is occupied, leading to internal displacement of residents. Data on development-induced displacement are limited but it is considered a serious concern in some of the most rapidly growing economies. For example, it has been estimated that 25-50 million Indians and 40 million Chinese have been internally displaced due to development projects in the past 50 years [11,12].
Finally, deportation, the state-ordered expulsion or voluntary departure under threat of expulsion of non-citizens for breaches of immigration or criminal law, is recognised as an increasingly important form of forced migration [13]. Rates of deportations are growing, particularly in Western countries. In the US, approximately 3.7 million 'removals' took place from 2003-2013, with the annual numbers of deportations doubling from 211,000 to 438,000 over this period [14]. Similarly, the number of people deported from the United Kingdom reached over 40,000 in 2011, a 46% increase since 2004 [15].

Health impacts of forced migration
There is some evidence that immigrant populations experience better health than native populations [16,17]. This 'healthy immigrant effect' has been attributed to both self-selection (educated, wealthy and healthy people are more likely to have opportunities to migrate) and exclusion of unhealthy migrants at immigration pre-screening. This is, however, unlikely to be the case for forced migrant populations, as forced migration is involuntary, commonly occurs on short notice, and impacts all classes of the community.
Forced migration can have diverse health impacts at all stages of the migration journey, including during transit, in countries of first asylum, and for those who are permanently resettled, in the receiving country. Health status varies across forced migration contexts. For example, IDPs may be more vulnerable to poor health than refugees due to their continued proximity to conflict zones, and limited access to services [18]. The health of forced migrants is also influenced by pre-existing health problems in the population prior to displacement, health system capacity in the location of asylum, and limited access to health services [19][20][21].
comorbidity among forced migrant populations [32][33][34], who may be particularly vulnerable to substance use as a coping mechanism.
Second, forced migrants experience acculturation challenges, the process of cultural and psychological change that follows contact with a culture other than one's own [35]. It has been hypothesised that migrants who are highly engaged in the host culture ('assimilation') may engage in substance use in order to adhere to mainstream norms and gain acceptance in their new communities. This may be a concern particularly in the context of resettlement in Western countries, where substance use, particularly alcohol consumption, is normalised. There is a dearth of literature on forced migrants specifically but a growing body of research, predominantly conducted among Hispanic populations in the US, has found a significant association between acculturation to dominant norms and substance use [36][37][38]. Acculturation is an especially important factor for younger migrants, whose experiences are compounded by intergenerational conflict, peer pressure and feeling caught 'between cultures' [36,[39][40][41]. Among young people, low levels of interest in maintaining their native culture alongside low levels of participation in their new culture, often due to discrimination and exclusion, has been associated with substance use [42].
Finally, forced migrants, particularly those resettled in Western countries, commonly experience social and economic inequality, marginalisation and discrimination [43][44][45][46]. These factors have all been shown to be important determinants of health [21,47,48], and may contribute to feelings of stress and powerlessness, which may in turn contribute to substance use. A recent systematic review examining the impacts of racial discrimination on health among children and young people found positive relationships in 60% of articles examining alcohol use as an outcome and 49% of articles examining drug use as an outcome [49]. In addition, forced migrants may be exposed to illicit drugs through residence in disadvantaged neighbourhoods where alcohol and drugs may be readily available [50,51].

Aims and significance of this review
With forced migration occurring at unprecedented levels, the potential for increases in substance-use related morbidity and mortality is concerning. Importantly, there are also substantial indirect health consequences of substance use in this vulnerable population, as it plays an important role in a number of other prevalent health conditions, including mental health disorders [28,32], gender-based violence [52,53] and infectious diseases such as HIV, hepatitis B and C and tuberculosis [54][55][56]. The negative consequences of substance use may also be exacerbated among forced migrant populations due to poor knowledge about substance use, stigma, and reduced access to health services [39,[57][58][59].
Despite being an emerging issue of global health significance, comprehensive review-level data examining substance use among forced migrant populations is lacking. Two reviews were conducted more than a decade ago [60,61], and two more recent reviews have been limited in scope, with one addressing alcohol only [62], and one examining substance use more broadly but only in conflict settings [63]. The current review aims to build on this previous work by consolidating the evidence on substance use among diverse forced migrant populations across camp, community and resettlement settings, and identifying priority areas for intervention and future research.

Methods
This review involved structured searches of peer-reviewed literature and was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (S1 File) [64]. No protocol for this systematic review has been published.

Research questions
Our review approach was guided by three overarching questions: What does available research tell us about the magnitude, and risk and protective factors for  substance use and related harms among forced migrants? 2. What are the contextual factors which underlie substance use among forced migrants?
3. What evidence is available to inform the design and implementation of interventions to address harmful substance use among forced migrants?
The research questions and our subsequent search strategy were developed using the SPI-DER tool, which has been designed for reviews incorporating qualitative and mixed-methods literature [65]. Our SPIDER parameters were: Sample-forced migrants; Phenomenon of Interest-substance use and related harms; Design-any; Evaluation-any; Research type-any.

Search strategy and eligibility criteria
Seven medical, allied health and social science databases (Ovid Medline, CINAHL, Ovid Psy-cINFO, Ovid Embase, Sociological Abstracts, International Bibliography of the Social Sciences, SocINDEX) were searched from inception to May 2015. Search terms, developed in consultation with a medical librarian, covered the key domains of forced migration and substance use (Domains S and PI of SPIDER), and were modified slightly for each database (S2 File). Articles were also identified through search updates conducted in September 2015, hand-searching reference lists of included articles and previous review papers, and contacting authors of identified conference abstracts. Archives of 15 migration, substance use and general public health conferences held between 2010 and 2014 were also searched for relevant abstracts, and corresponding authors were contacted to enquire whether any related peer-reviewed publications were inpress or recently published. One further eligible paper was identified by an anonymous reviewer during the manuscript peer-review process. Literature searching was managed using Mendeley (Mendeley Ltd, 2015).
Studies were considered eligible for inclusion in the review if they described any findings related to alcohol or illicit drug use among forced migrant populations (defined using the IOM definition [1]). Relevant findings considered included: prevalence or frequency of use, prevalence of hazardous/harmful use or dependence (including self-reported), analysis of factors associated with substance use, substance use service provision, demand or utilisation, lived experiences of forced migrants who use substances, exposure to substance use, and engagement in the production or sales of alcohol or illicit drugs. Quantitative and qualitative studies involving forced migrant populations, as well as studies involving key experts or stakeholders working with forced migrant populations were included.
Articles were excluded if they were not original peer-reviewed research, were not published in English, French or Spanish, did not clearly identify the population as forced migrants or did not provide separate results for forced migrants, or did not report any relevant findings. Multiple reports from the same study were excluded unless they reported additional relevant data.

Screening, selection and data extraction
Following an initial screen of titles and abstracts, potentially relevant articles were selected for full text review. A random 10% of full-text articles were checked by a second reviewer and showed high inter-rater agreement on inclusion/exclusion decision (Cohen's Kappa DH/JM: 0.82, DH/RF: 0.82).
From included articles, article publication characteristics (e.g. year of publication, journal), study procedures (e.g. study design, participant recruitment methods, data collection methods), participant characteristics (e.g. type of forced migrant population, socio-demographic characteristics) and substance use findings were extracted into a purpose-designed Microsoft Access database by JM and RF, and independently reviewed by DH.

Quality assessment
A quality assessment was conducted using the Critical Appraisal Skills Programme (CASP) checklists for cohort studies, case-control studies and qualitative studies [66]. The checklist for cohort studies was modified for application to cross-sectional and case studies (e.g. Question 2, 'Was the cohort recruited in an acceptable way?" was modified to 'Was the sample recruited in an acceptable way?', and questions regarding follow-up of participants were excluded). Quality assessment focused on assessing strengths and weaknesses of each study; a total score was calculated for each study based on relevant checklist items and then a grade of low, moderate or high was assigned through discussion between two authors (DH and JM). For mixed methods studies, separate scores were calculated for the quantitative and qualitative components, with one overall grade assigned.

Data synthesis
Due to the heterogeneity of study designs, populations and outcome measures, a meta-analytical approach was considered inappropriate. Synthesis of evidence from quantitative studies was descriptive; results presented focus primarily on studies which used validated measures. 95% Confidence Intervals for prevalence estimates were calculated using Stata 13.1 (Statacorp LP, Texas, USA). Synthesis of evidence from qualitative studies focused on identifying prominent themes relating to the contexts and consequences of substance use.

Study characteristics
A total of 63 relevant articles were included in the review (Fig 1). The most common reason for exclusion at full-text review was that the study population could not be clearly identified as forced migrants (n = 116, 36% of articles reviewed; Fig 1).
The majority of included articles examined substance use among refugee, IDP and asylum seeker populations (n = 55, 87%; Table 1), with a small number of studies conducted among people displaced by natural disasters (n = 4, Table 2) and deportees (n = 4, Table 3). No studies were identified among populations displaced by man-made disasters or development. Over two thirds of studies among refugees, IDPs and asylum seekers (n = 38, 69%) were conducted in high-income countries, predominantly in the US (n = 17) and Central Europe (n = 12). Of the 17 studies conducted in low and middle-income countries, 41% were conducted in Asia (n = 7), 24% in Sub-Saharan Africa (n = 4), and the remainder in Latin America, Eastern Europe and multiple country settings. All four studies involving people displaced by disasters were conducted in the US, and all four studies involving deportees were conducted in Mexico. Nine of the 51 studies which stratified by gender included samples which were exclusively male (18%), and a further 12 studies (24%) comprised at least 60% males. Only three studies focused specifically on children or young people. Just over half of all identified studies were published in the past five years.

Study types and quality assessment
Three quarters of studies employed quantitative methods (n = 44), 16 studies utilised qualitative methods and three studies used mixed-methods. The majority of quantitative studies used cross-sectional study designs (82%). Half of the quantitative studies used probability sampling methods (n = 22, 47%) and almost two thirds (n = 28, 60%) included samples of 200 or more participants. Sixteen studies included a comparison sample of native-born or non-forced migrants. Qualitative studies employed a range of methods, including participant interviews and focus groups, key expert interviews and ethnographic methods. Ten studies were rated as high methodological quality (16%; 8 quantitative and 2 qualitative studies), 39 as moderate quality (62%) and 14 as low quality (22%).

Synthesis of findings
Prevalence of alcohol use and dependence. Studies collected alcohol use prevalence estimates over the lifetime (n = 4), past year (n = 2) and past month (n = 4). Lifetime alcohol use prevalence estimates were all among refugee communities in the US and ranged from 13% among Iraqi refugees [83] to 38% among Cambodian refugee women [90]. Past-month alcohol use ranged from 26% among Cambodian refugees in the US [89] to 56% among high-school aged Serbian IDPs [75]. Studies also reported on prevalence of binge drinking, alcohol-related "problems", having "trouble with alcohol" and "excessive alcohol consumption", but definitions for these measures were not provided. Only one study used longitudinal methods to examine changes in alcohol use over time, finding that the prevalence of lifetime alcohol use among newly-arrived Iraqi refugees in the US increased from 20% to 39% over a 12-month period [84].
Twelve studies measured the prevalence of hazardous/harmful alcohol use, of which six collected this information using validated measures, all using the Alcohol Use Disorders  -14% reported having trouble with drugs sometimes. 40% reported using alcohol to cope with sorrows or problems and 12% used drugs to cope with sorrows or problems.  Locating drugs was a major concern for many women immediately after deportation, with some women exchanging sex for drugs. The 4 women who rarely or never injected drugs in the USA began injecting regularly following deportation. Women described beginning to inject because of drug dependence, lack of self-control, and the influence of their social networks and neighborhoods. Although nearly all participants described wanting to "get clean" few had attended drug treatment programs in Tijuana, with financial access identified as a major barrier to treatment.
Cross-sectional study. Recruited from ongoing PWID cohort study, which recruited participants using respondentdriven sampling. Intervieweradministered questionnaire. Identification Test (AUDIT [127]). In camp settings, the prevalence of hazardous/harmful alcohol use ranged from 17-36%, but was as high as 66% when measured among past-year drinkers only [78] (Fig 2). In community settings, the prevalence was 4-7% overall, and 14-19% among current drinkers. One further study used an adapted single-item measure based on the third question of the AUDIT, finding that <1% of women attending the antenatal care clinic at a refugee camp in Thailand reported 'risky' alcohol use [70]. Fig 3 shows the prevalence of alcohol dependence/alcohol use disorder assessed in 12 studies using validated measures (AUDIT, Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria). In camp settings, the prevalence ranged from 4% among Burmese refugees in Thailand [69] to 42% among IDPs in Croatia [33]. Among IDPs in Uganda, 4% of the total sample and 17% of past-year drinkers recorded AUDIT scores of 20 or higher, suggesting a need for alcohol treatment [78]. In community settings, the prevalence of past-year alcohol dependence/use disorder ranged from less than 1% among Somali refugees in the UK [86] to 25% among IDPs in Croatia [88]. Few studies examined prevalence of alcohol use, hazardous/ harmful use or dependence/use disorder stratified by age or sex.
Prevalence of drug use and dependence. Only two studies reported on prevalence of drug use. Among a sample of Afghan refugees in Pakistan, 23% had ever used drugs and 7% had ever injected drugs [73] and among Somali refugees in the US, 44% reported any drug use [99]. No studies reported validated measures of prevalence of hazardous/harmful drug use, but six studies, all involving refugee populations, measured the prevalence of drug use dependence/ disorder using validated measures (Drug Use Disorders Identification Test (DUDIT), DSM  criteria; Fig 4). Prevalence was below 5% among the five studies conducted in community settings, but was 20% in the one study conducted in a camp setting [67]. None of these studies provided information about the types of drugs used among those with drug dependence.
A small number of studies reported findings related to specific drug types, most commonly cannabis, cocaine, heroin and methamphetamine. One study used the Alcohol Use Disorder and Associated Disabilities Interview Schedule-IV to measure lifetime prevalence of substance use disorders among refugees in the US, finding a prevalence of 4% for cannabis use disorder, and below 1% for cocaine, hallucinogens, amphetamines, and opioids/heroin use disorder [111]. Among disaster displacees in the US, some increased use of marijuana, ecstasy and methamphetamine was reported, with changes in patterns of drug use post-displacement linked to changes in drug availability [119,122]. Among a sample of people who inject drugs in Mexico, 16% reported trying new drugs post-deportation, most commonly heroin [126]. Cannabis, cocaine, and to a lesser extent amphetamines, were noted as emerging drugs of concern in refugee camp settings [52,81]. In Norway, an ethnographic study identified cannabis dealing as an important source of income for newly-arrived refugees who had limited cultural capital and work and educational opportunities [112].
Comparisons with non-forced-migrant populations. Of the 16 studies which included a comparison sample of non-forced migrants, only seven conducted statistical analysis examining differences in alcohol or drug use outcomes (Table 4). In the US, refugees were significantly less likely to report alcohol use, injecting drug use [85] and alcohol use disorder and most substance use disorders compared with non-refugees [111]. In Sweden, a population-based study found refugees were significantly less likely to experience an alcohol-related hospital admission compared with native-born, and while refugee women were significantly less likely, refugee men were significantly more likely than their non-refugee counterparts to experience a drugrelated hospitalisation [114]. No statistically significant differences were found in binge drinking among Vietnamese refugees in the US, compared with returnees and never-leavers [93] or when comparing excessive alcohol consumption among IDP and non-IDP slum residents in Colombia [77].
Two studies compared patterns of drug use and related risk behaviour between drug-using forced migrant and non-forced migrant samples. No significant differences in the proportion of participants reporting opiates as the first drug injected or reporting current injecting drug use was found between Afghan refugees and native Pakistanis [82]. Among people who inject drugs in Tijuana, Mexico, some differences in patterns of drug use were detected between deportee and non-deportee participants [123]. A noteworthy finding of this study was that  Table 4. Findings from seven studies presenting analyses comparing alcohol or substance use outcomes between forced migrants and a non-forced migrant sample. Reference  deportees were significantly less likely to report ever having received drug treatment or HIV testing compared with non-deportees who inject drugs.
Correlates of substance use. Seven studies conducted statistical analysis to identify correlates associated with alcohol outcomes, including current/past-month drinking, increasing alcohol & tobacco use post-disaster, and hazardous/harmful drinking (Table 5). In five of these analyses, male sex was positively associated with the outcome [74,78,89,102,119], and in three trauma exposure and/or symptoms of mental illnesswere positively associated [78,79,100].
Studies examining correlates of drug use outcomes were limited to two studies of regular drug users, one among deportees and one among people displaced by a hurricane (Table 6). Factors identified as being significantly associated with substance use outcomes (increasing substance use post-disaster and new drug use post-deportation) included experiencing greater disaster damage, greater numbers of deportations and feeling sad or depressed [119,126].
Interventions to address substance use. Only one intervention study was identified-a screening and brief intervention for high-risk alcohol use in a refugee camp setting in Thailand [69]. Although feasibility of screening in this setting was established, uptake of referrals to specialist treatment was low (6%).
Other notable findings. Substance use was commonly examined in the context of mental health. Two case studies described use of drugs including opiates, cocaine and cannabis to selfmedicate symptoms of PTSD among refugees and asylum seekers [34,98], and seven qualitative studies identified coping with trauma and loss as reasons underlying the use of alcohol and drugs [52,71,81,92,95,108,122]. Three studies examined the association between khat use (a mild stimulant native to the Horn of Africa) and mental health outcomes among Somali refugees, with mixed findings [86,87,99]. One study found that post-disaster substance use was associated with a three-fold risk of major depressive disorder among hurricane displacees in the US [121]. An assessment of 180 services providing mental healthcare and substance abuse treatment across a number of European settings found that only 10% provided specific programs for refugees and/or asylum seekers [115]. Similarly, limited access to drug and alcohol information and services was identified as a key factor associated with substance use and mental health comorbidity for refugee young people in Australia [108].
Several studies focused on substances that were traditionally used in migrants' countries of origin. Among the Burmese refugee community in Wollongong, Australia, betel quid chewing was seen as a benign social habit [94]. Similarly, khat chewing was considered a common and socially acceptable pastime among East African communities, but was also used to cope with stress and feelings of hopelessness [68,81]. Several studies examined opium use among South-East Asian refugees, finding that fewer refugees had accessed drug use treatment compared with American-born samples [117], and that family and cultural pressures were identified as reasons for discontinuing treatment [101].
Eleven studies examined the unique context of refugee camp settings. Within these studies the important role of alcohol production and sales as a source of income for these vulnerable communities was acknowledged [52,81]. Within these settings, alcohol was also implicated in a range of harms, including gender-based violence [52,71] and neglect and violence towards children [76].

Key findings and implications for public health
Our global systematic review shows that despite substantial growth in research examining substance use among forced migrants in recent years, the available body of evidence remains limited. On the basis of the available evidence however, we draw attention to several findings with important implications for public health. A major finding of this review is that there is substantial heterogeneity in patterns of substance use across included studies. In studies using validated measures, the prevalence of hazardous/harmful alcohol use ranged from 4%-36%, alcohol dependence ranged from <1%-42% and drug dependence ranged from 1%-20%. Further, among studies including comparison samples, some found lower rates of substance use among forced compared with non-forced migrant samples, some found no statistically significant differences, and one study detected associations in different directions among male and female participants. The heterogeneity in findings likely reflects regional and global differences in patterns of substance use, which may be influenced by local context factors such as availability of substances and social norms [128,129]. This level of heterogeneity limits our capacity to draw accurate conclusions about substance use among forced migrants, but supports our belief that substance use is an issue of increasing public health significance among these populations.
A particularly notable finding is that if we consider the upper limits of these estimates, as many as one in three forced migrants may be using alcohol in harmful or hazardous ways, and, when measured among current drinkers only, this estimate may be as high as two in three. Importantly, these figures may actually underestimate the true prevalence of harmful drinking as stigma has been shown to influence non-response and underreporting of substance use among culturally diverse communities [130][131][132][133]. In particular, the prevalences of both hazardous/harmful and dependent drinking identified in our review generally tended to be higher among samples in camp settings compared with community settings, suggesting that camp settings may present a particularly vulnerable risk environment for substance use. These findings indicate a need to integrate substance use prevention and treatment into services offered to forced migrants, particularly in camp settings. Currently, the Sphere Handbook, which outlines minimum standards of provision in humanitarian response, includes only a brief mention of substance use, recommending that "people have access to health services that prevent or reduce mental health problems, including minimising harm related to alcohol and drugs" [134]. We recommend that these guidelines be expanded to provide specific actions and targets for not only minimising harm in these settings, but also addressing the underlying factors which may lead to substance use. Informed by recently published World Health Organisation (WHO) recommendations for evidence-based responses to substance use in low and middle-income countries and humanitarian settings, this should include incorporating screening, brief intervention and brief motivational conversations into routine clinical practice and increasing availability of psychosocial support and medication-assisted therapies for substance use dependence and withdrawal [135,136]. It will be important for substance use services to be integrated with mental health services, and to employ a trauma-informed care perspective which focuses on respectful and empowering practices to minimise re-traumatisation [137,138]. A second key finding of this review relates to substance use among resettled forced migrants in Western settings. Although the prevalences of alcohol and illicit drug use tended to be relatively low in studies included in this review, there was substantial variation in the recorded estimates and the time points post-migration at which these measurements were captured. Given the high rates of alcohol use occurring in refugee camp settings, it is likely that substance use among newly-arriving refugees may be occurring at higher levels than what is being detected. Although current evidence-based clinical guidelines for screening and treating newly-arrived immigrants and refugees do not specifically address substance use we suggest that, consistent with the recommendations for PTSD and intimate partner violence, clinicians should be alert for signs and symptoms of substance use, and investigate further where appropriate [139]. Given another key finding of this review was that, as in the general population [128], male sex and experiences of trauma and poor mental health are associated with substance use among forced migrants, we recommend that clinicians should be particularly alert for signs of substance use among these groups.
A final key finding is that the single longitudinal study included in this systematic review detected a statistically significant increase in the prevalence of lifetime alcohol use among newly-arrived refugees in the US over a 12-month period [84], suggesting that vulnerability to substance use may increase following resettlement. This is consistent with other research has found that migrant health decreases over time due to a range of factors including resettlement challenges, acculturation and barriers to health service use [140][141][142]. This finding underscores the importance of maintaining contact with nearly-arrived forced migrants to monitor changes in substance use during the early post-migration period.

Limitations of existing research
Earlier reviews of substance use among displaced persons identified methodological issues such as small sample sizes, use of non-probability sampling methods and lack of comparison groups as major limitations of existing research [62,63]. The quality of literature identified in this updated review indicates some improvement, particularly in relation to sampling methods and use of validated measures (just under one fifth of included studies were of high methodological quality), however some concerns persist.
First, the representativeness of study samples is limited. With the overwhelming focus on refugee populations, knowledge of substance use among persons displaced due to disasters, development and deportation remains extremely limited. These groups may represent a particularly vulnerable subset of forced migrants as they commonly lack the protections and supports that are afforded to refugees and IDPs under internationally recognised treaties. Moreover, current research has focused predominantly on resettled refugees in high-income settings, and findings may not be generalisable to refugee populations in low and middleincome settings, which is a major concern given that over 80% of the global refugee population resides in these settings [4]. Importantly, studies conducted in low and middle-income countries have focused almost exclusively on camp settings; as refugees/IDPs in urban settings now outnumber those in camp settings [5], this is an important group to consider in future research. In addition, across both displacement and resettlement settings, young people and women have been under-represented among refugee, IDP and asylum seeker samples studied. Young people are a particularly important sub-group as evidence shows that alcohol and illicit drug use are leading causes of morbidity and mortality among young people [143,144]. Although women generally report lower rates of substance use compared with men they are important to include in research as they are at increased risk of developing alcohol-related health consequences and are also more vulnerable to experiencing harms from others' drinking [70,145,146]. Second, there is a dearth of studies using longitudinal methods, which limits our understanding of how substance use changes across the migration trajectory including following resettlement. Third, few studies have included appropriate control groups, making it impossible to assess how patterns of substance use differ between forced migrants, other migrant groups and native populations. Fourth, although some studies have used multivariable analysis to identify risk factors for substance use, these analyses have tended to focus on individual-level characteristics, and there has been limited consideration of potentially important structural factors such as acculturation. Finally, the paucity of intervention studies severely limits the evidence base for responding to substance use issues among forced migrant populations.

Limitations of this review
The heterogeneity of included studies in terms of populations, settings and outcome measures precluded pooling of data in a meta-analytic manner, and makes it difficult to draw definitive conclusions. The search strategy used focused specifically on substance use. It is possible that articles broadly examining the health or mental health of forced migrants which included substance use measures but did not include substance use terms in the article title, abstract or key words may not have been identified. The review was limited to studies published in English, French, or Spanish. Although few articles were excluded due to this eligibility criteria, it is possible that findings in excluded studies were in a different direction from those included, potentially biasing the results. This review was also limited to peer-reviewed published literature as it was not feasible to conduct grey literature searches due to limited resources. Finally, a large number of sources were reviewed by a small research team in a short period of time, and although internal checks were conducted to minimise errors in data coding and entry, some unintended errors may have occurred.

Recommendations for future research
Many important questions regarding substance use among forced migrants remain unanswered. Further research is needed to robustly assess the burden of substance use and dependence in low and middle-income countries, as well as among other sub-groups of forced migrants who have been under-studied, such as people displaced by disasters, and deportees. Importantly, as the bulk of existing research has focused on alcohol use, there is a need for additional research examining illicit drug use, including measures of individual drug types rather than unspecified 'drug use', and examination of transitions between different routes of drug administration. Multi-country studies which incorporate standardised recruitment, data collection and analysis across diverse settings (such as the work done on intimate partner violence in Asia and the Pacific [147]) would be an appropriate approach to producing robust data which will be informative at the site-specific level, as well as enabling effective comparisons to be made across different contexts.
In addition to measuring prevalence and identifying risk factors for substance use, it is essential to conduct research to identify factors which are protective against substance use. Drawing on the broader epidemiological literature, particularly research conducted among migrant and minority communities, factors warranting exploration include individual-level factors such as agency, resilience, and coping skills, community-level factors such as community cohesion, social network systems, and norms about substance use, environment-level factors such as availability of alcohol and illicit drugs, and policy-level factors such as rights to obtain citizenship, and legal, economic and social integration [148][149][150][151]. Better understanding of these factors may inform broad social and economic interventions which may have downstream effects on health and wellbeing [152]. Future research is also needed to explore forced migrants' knowledge about substance use, experiences of health and social consequences of substance use, and factors affecting perceived need for, access to, and uptake of care. Together, this information will be crucial to addressing key policy objectives of the WHO in relation to the health of migrants, namely evidence-based program and policy development, and developing migrant-friendly health systems and services [152].
Future research should be designed to ensure inclusion of under-represented groups at high risk of substance use, such young people, and should employ longitudinal methods, in order to examine changes in substance use at different time points in the migration and resettlement trajectory. In particular, prospective cohort studies are crucial in identifying new-onset substance use and changes in individuals' patterns of substance use over time. Although there are inherent challenges in conducting cohort studies with migrant populations, such as ongoing insecurity and population mobility which may impede follow-up, strategies which have had some demonstrated success include employing dynamic and adaptive methodological approaches, collaboration with established services, ongoing community engagement and utilising the capacity of information and communication technologies [153][154][155][156]. Repeated crosssectional studies may also be valuable in resource-constrained settings such as refugee camps; although they cannot measure trajectories of substance use within individuals, they may be a meaningful way of monitoring population-level trends in substance use within a specific setting, in the same way they have been used to monitor trends in infectious disease prevalence [157,158].
Most importantly, the major gap in knowledge regarding effective, culturally competent interventions to prevent substance use and reduce related harms must be addressed. Recent research has noted that brief community-based interventions for alcohol use disorder have been shown to be effective in high-income settings [159]; explorations of how such interventions could be adapted for low and middle income settings and for substances other than alcohol would be a useful starting point. Several systematic reviews have identified family and community support as promoting resilience and improving mental health among forced migrants [160][161][162]; these may be important components for consideration in future interventions addressing substance use also. A potentially useful methodological approach for future intervention studies is the stepped wedge randomised controlled trial (RCT), in which groups are randomised to receive the intervention at different time points, with all groups eventually receiving the intervention. Stepped wedge RCTs are being increasingly used to address some of the logistical and ethical concerns arising in traditional RCTs (e.g. denial of a potentially effective intervention), and have been successfully implemented in low and middle-income country settings [163,164].

Conclusions
In summary, findings from this systematic review draw attention to our limited understanding of the epidemiology of substance use among forced migrant populations, particularly among persons displaced due to disasters, development and deportation, as well as among refugees in low and middle-income countries, where over 80% of the global refugee population resides. Findings suggest a need to integrate substance use prevention and treatment into services offered to forced migrants, particularly in camp settings. Given the recent and continuing projected increases in forced migration globally, longitudinal research to improve our understanding of substance use across the migration trajectory, and efforts to develop and evaluate interventions to reduce substance use and related harms are needed.