Skip to main content
Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Vitamin B12 Status in Metformin Treated Patients: Systematic Review

  • Qilin Liu ,

    Contributed equally to this work with: Qilin Liu, Sheyu Li

    Affiliation Department of Endocrinology and Metabolism, West China Hospital of Sichuan University, Chengdu, China

  • Sheyu Li ,

    Contributed equally to this work with: Qilin Liu, Sheyu Li

    Affiliation Department of Endocrinology and Metabolism, West China Hospital of Sichuan University, Chengdu, China

  • Heng Quan,

    Affiliation Department of Endocrinology and Metabolism, West China Hospital of Sichuan University, Chengdu, China

  • Jianwei Li

    Affiliation Department of Endocrinology and Metabolism, West China Hospital of Sichuan University, Chengdu, China



Randomized controlled trials and observational studies have yielded inconsistent results on the effects of metformin on vitamin B12 reduction. We therefore performed a systematic review to analyze the effects of metformin on vitamin B12 concentration.


PubMed, Medline, Embase, and the Cochrane central registry of controlled trials were searched to identify randomized controlled trials and observational studies exploring the association between metformin and vitamin B12 concentration in patients with type 2 diabetes mellitus or polycystic ovary syndrome. The main outcome measure was changes in serum vitamin B12 concentration after 6–208 weeks of treatment with metformin, as compared with placebo or other anti-hyperglycemic therapy.


Six randomized controlled trials met the inclusion criteria. Serum vitamin B12 concentrations were significantly lower in patients treated with metformin than in those who received placebo or rosiglitazone (mean difference [MD], −53.93 pmol/L; 95% confidence interval [CI], −81.44 to −26.42 pmol/L, P = 0.0001). Subgroup analysis identified four trials in which patients received a lower dose of metformin (<2000 mg/d) and two in which they received a higher dose (≥2000 mg/d), with MDs in vitamin B12 concentration after metformin treatment of −37.99 pmol/L (95% CI, −57.44 to −18.54 pmol/L, P = 0.0001) and −78.62 pmol/L (95% CI, −106.37 to −50.86 pmol/L, P<0.00001), respectively.


The reduction of vitamin B12 may be induced by metformin in a dose dependent manner.


Metformin is now the most widely used antidiabetic drug, with almost all guidelines throughout the world recommending metformin as first-line treatment for patients with type 2 diabetes mellitus (T2DM). Metformin may also be used to treat other conditions involving insulin resistance, such as polycystic ovary syndrome (PCOS) [1]. Metformin has beneficial effects on carbohydrate metabolism, weight loss, and vascular protection [2], but also has important side effects. For example, patients on long-term metformin therapy were found to be at risk of anemia [3]. This may be due to a metformin related vitamin B12 reduction. It is reported that, 30% of patients receiving long-term metformin treatment experienced malabsorption of vitamin B12, with a decrease in serum vitamin B12 concentration of 14% to 30% [4].

Vitamin B12 is a vital nutrient for health. It plays an important role in the functioning of the brain and nervous system, and in the formation of red blood cells. In addition to anemia, vitamin B12 deficiency may increase the severity of peripheral neuropathy in patients with T2DM [5]. Furthermore, because vitamin B12 participates in the most important pathway of homocysteine (Hcy) metabolism, a reduction in vitamin B12 would increase plasma concentrations of Hcy, which is strongly linked to cardiovascular disease in patients with T2DM [6] and PCOS [7].

Although some clinical studies have reported that metformin lowered vitamin B12 level, other studies have reported that it did not. To date, no consensus has been reached on whether metformin induces vitamin B12 reduction. We therefore performed a meta-analysis to assess the association between metformin treatment and vitamin B12 reduction.


This systematic review was planned, conducted, and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement (Table S1) and the Cochrane handbook for Systematic Reviews of Interventions [8], [9].

Inclusion criteria

We included studies that met the following criteria: (1) It should include patients with T2DM or PCOS who met strict diagnostic criteria and did not take B group vitamins prior to entrance into the studies. (2) For randomized controlled trials (RCTs), patients should be randomized to treatment with metformin or to placebo or another hypoglycemic drug and the groups compared by valid statistical methods. For observational studies, a group treated with metformin should be compared with another group treated with placebo or other antidiabetic drugs. (3) The main outcome should be change in serum vitamin B12 concentration. (4) All studies should report factors associated with changes in vitamin B12 levels necessary to perform a meta-analysis or sufficient information to estimate it.

Exclusion criteria

Studies without available data, duplicate publications, and studies in a language other than English were excluded.

Search strategy

The PubMed, Embase, and Cochrane central registry of controlled trials were systematically searched for all papers published through October 2013. Subject headings were combined with keywords and their synonyms, using search terms such as ‘metformin’, ‘Glucovance’, ‘dimethylbiguanid’, ‘vitamin B12’, ‘B12’, and ‘cobalamin’. References in selected articles and published reviews were also manually searched. Literature searches were performed independently by two investigators, with discrepancies resolved by group discussions. Additional studies and missing information in published reports were searched via direct author contact. Complete search strategy is reported as Appendix S1.

Validity assessment

The validity of the eligible RCTs were evaluated in accordance with the Cochrane Collaboration guidance [8] which includes the following criteria: (1) random sequence generation, (2) allocation concealment, (3) blinding of participants and personnel, (4) blinding of outcome assessment, (5) incomplete outcome data, (7) selective reporting, and (8) other bias. For each criterion, an answer of ‘Yes’ indicated low risk of bias, ‘No’ indicated high risk of bias, and ‘Unclear’ indicated either lack of information or uncertainty over the potential for bias. We also applied the Newcastle-Ottawa Scale (NOS) [8], [10] to assess the quality of the included observational studies.

Data extraction

Titles and abstracts were screened to identify clinical trials and observational studies. Full text articles of studies that fulfilled the inclusion criteria were obtained. Data obtained from each article included its title, author names, year of publication, study design, participant characteristics, and endpoint data. A meta-analysis was performed to examine the association between metformin treatment and the changes of vitamin B12 concentration. Vitamin B12 concentrations and changes were measured as pmol/L. Heterogeneity among the studies was evaluated using chi-square test and I2 statistics, with P≤0.1 and I2>50% indicating heterogeneity. A fixed effects model was selected for non-significant heterogeneity, or a random effects model for heterogeneity. The mean difference (MD) in each study was determined. The MDs were combined, and the pooled MDs along with their 95% confidence intervals (CIs) were calculated using Review Manager (RevMan, version 5.2). Subgroup analyses were performed by separating studies according to comparators, follow-up time, and background treatment. Tests for overall effect were assessed using z-statistics.


Search results

A total of 679 articles were identified during our search. After screening of titles and abstracts, 43 met the criteria of our review, which were aimed at determining the influence of metformin on vitamin B12 status (Figure 1). Of these, 10 articles were RCTs and 33 were observational studies. Of the 10 RCTs, three reports were excluded because both groups received metformin as background treatment, and one was excluded because no major indices could be extracted, and eventually only six randomized controlled trials [11][16] met the inclusion criteria. Among the 33 observational studies, 15 were conference abstracts without available full texts [17][31] which could not provide sufficient information for quality assessment or data analysis, and 14 cross-sectional studies [3], [32][44] and 1 case-control study [5] lacked data of changes in serum vitamin B12 concentration from baseline, but vitamin B12 concentration changes had been defined as the primary outcome of this study. Thus, 30 studies were excluded. And the other 3 observational studies were cohort studies. One study compared metformin with phenformin [45], one study did not give vitamin B12 concentration changes [46], and the other one study enrolled the patients taken B group vitamins supplements [47]. Therefore, none of the observational studies met the inclusion criteria.

Study characteristics

The characteristics of the six RCTs included in the final meta-analysis are shown in Table S2. All six were published between 2000 and 2010. Overall, they included 6 cohorts and 816 participants, with 610 completing the studies. The mean ages of the participants ranged from 24.1 to 64.0 years. The mean follow-up duration ranged from 6 to 208 weeks. Quality assessment of these studies by the Cochrane Collaboration Risk of Bias Tool is shown in Figure S1. All of the 6 RCTs were adequate randomized without selective reporting and other biases. Allocation concealment of 2 studies [11], [15], blinding of outcome assessment of 3 studies [13][15], and incomplete outcome data of 1 study [15] were all not described in details. In addition, blinding of participants and personnel was probably not been done in 2 studies [13], [15].

Metformin and vitamin B12 reduction

Overall, metformin had a significant effect on vitamin B12 concentration when compared with other interventions (MD, −53.93 pmol/L; 95% CI, −81.44 to −26.42 pmol/L, P = 0.0001; Figure 2). However, we detected significant heterogeneity among the studies. The two trials [13], [14] with the lowest weight (total weight 9.3%) did not show a significant effect. The funnel plot was limited as only six trials were included.

Figure 2. Effects of metformin on compared with other interventions vitamin B12 concentration.

As each 1000 mg/d metformin dose increment increased the risk of developing vitamin B12 deficiency [4], we chose metformin 2000 mg/d as the cut-off point in our subgroup analysis. The four trials [11], [13][15] belonging to the lower dose group showed that metformin had a significant effect compared with other interventions (MD, −37.99 pmol/L; 95%CI, −57.44 to −18.54 pmol/L, P = 0.0001), without any heterogeneity. The other two trials [14], [16] belonging to the higher dose group also showed a significant effect (MD, −78.62 pmol/L; 95%CI, −106.37 to −50.86 pmol/L, P<0.00001), with significant heterogeneity. We found that vitamin B12 concentration was reduced more in the higher than in the lower dose group, indicating that the reduction of vitamin B12 was associated with metformin dose.

We also assessed subgroups based on differences in duration of metformin use, background therapy, control group treatments, diseases of participants, countries, methods of B12 measuring and qualities of RCTs (Table S3). Subgroup analysis by diseases of participants showed that the effect of metformin on vitamin B12 concentration was nearly the same in patients with T2DM and PCOS. Moreover, subgroup analysis also indicated that metformin reduced vitamin B12 concentration in both long (≥3 years) and short (<3 years) treatment duration subgroup. All subgroups analysis according to any parameters, which may lead to heterogeneity, showed similar effects of metformin on vitamin B12 reduction.

Adverse events

Two studies of RCTs didn't report any adverse event [11], [15]. Among the other 4 studies, gastrointestinal side effects were the most common adverse events [13], [14], [16]. In Carlsen's research of infertile PCOS women, the incidence of minor gastrointestinal side effects was 55.6% in the metformin group but 13.5% in the placebo group [14]. In his study of pregnant PCOS women, nausea and gastrointestinal discomfort was also found in 17.6% patients treated with metformin and 14.3% patients treated with placebo [14]. In Kilicdag's study in T2DM patients [13], none of the patients with rosiglitazone reported any adverse effects, but about 20% patients treated with metformin had problems of nausea and vomiting. De Jager et al. [16] found 46 of 390 T2DM participants (30 with metformin, 16 with placebo) experienced adverse events such as diarrhea, flatulence, fatigue, pruritus, headaches, heartburn and nausea. Additionally, 11.3% patients with metformin but 5.6% patients with placebo reported a history of diarrhea. However, there was no significant difference in the incidence of other side effects. In general, gastrointestinal side effects were more common in patients treated with metformin. Adverse events of the 6 RCTs were shown in Table S4.


Overall, we found that metformin treatment of patients with diabetes and PCOS resulted in statistically significant reductions in vitamin B12 concentration. Although none of the 33 observational studies met the inclusion criteria, almost all of these observational studies supported that metformin exposure was associated with a significant reduction of vitamin B12. For example, Kos' cohort study [47] found vitamin B12 levels of T2DM patients treated with metformin for more than 4 years were significantly lower than control (MD, −152.2 pg/mL; 95%CI, −220 to −84 pg/mL, P<0.0001). And Greibe's cohort study [46] demonstrated that, compared with placebo, serum vitamin B12 in women with Polycystic Ovary Syndrome treated with metformin was decreased after 6 months of treatment. Besides, cross-sectional studies and surveys [41], [44] also showed a significant effect of metformin therapy on vitamin B12 reduction, when compared with other hypoglycemic therapy.

In consistent with the observational studies, meta-analysis of RCTs also found that metformin reduced vitamin B12 levels in patients with diabetes and PCOS. Subgroup analysis showed higher-dose metformin could reduce vitamin B12 concentration more significantly. Our meta-analysis included RCTs of patients with PCOS in addition to T2DM, because both diseases have a common pathogenesis, i.e., insulin resistance. Subgroup analysis showed that metformin had nearly the same effects on vitamin B12 in patients with T2DM or PCOS.

Because metformin, which delays glucose absorption, has an effect on small bowel motility and on bacterial overgrowth [48]. According to our review, gastrointestinal adverse effects were the frequently observed in patients treated with metformin. Metformin-induced B12 malabsorption may be due to digestive changes, which leads to the binding of B12-intrinsic factor (IF) complex and a reduction of B12 absorption [49]. Since the B12-IF complex binds to the ileal cell surface receptor, metformin alterations in IF levels and/or ileal morphological structure may lead to B12 reduction [50], [51].

The clinical significance of biochemical change in serum vitamin B12 concentrations remains controversial. Some previous studies showed that lower serum B12 concentration caused by metformin within the normal range could be clinically meaningful. A Greek cohort including 600 diabetic patients showed association between metformin prescription and vitamin B12 dependent megaloblastic anemia [3]. Cognitive impairment and the progression of diabetic peripheral neuropathy may also be accelerated by metformin in a vitamin B12 dependent manner [52]. So, it was implied that the decrease of serum B12 concentration within the normal range should not be overlooked.

However, some studies showed that metformin may even improve B12 metabolism [43], or only decreases the inactive form of vitamin B12 (known as holo-haptocorrin, holoHC), rather than the biological active form (known as holo-haptocorrin, holoTC) [46], [53]. On the other hand, some other studies just provided the opposite results [5], [35] and the lowered serum level of vitamin B12 caused by metformin was reported to be clinically meaningful.

Different countries have different eating habits and nutrition supplements. And prevalence of vitamin B12 deficiency varies with population and B12 cut-off used. According to a national survey from U.S., consumption of routine vitamin B12 supplement may not rescue the biochemical B12 reduction in diabetic patients with metformin usage, suggesting that the recommended amount of vitamin B12 by the Institute of Medicine (IOM) may not be sufficient in those using metformin [41]. However, several studies [54], [55] found that treatment with vitamin B12 in a higher dose could reverse some of these disadvantages and vitamin B12 deficiency. Therefore, the supplement of vitamin B12 may be reasonable, and the dose should be individualized according to different ethnicity and habits.

Our study has several limitations. First, although we included patients with PCOS or T2DM, it is not clear whether these diseases cause vitamin B12 reductions independently. Second, the examining methods of vitamin B12 concentration were diverse in the included studies. Third, we only searched for articles published in English, while studies published in other languages could not be analyzed according to our knowledge. Moreover, potential publication bias could be introduced due to negative results may be rejected by journals. Finally, we did not analyze confounding factors that may introduce heterogeneity, such as obesity and age.

In conclusion, our meta-analysis showed that metformin could reduce vitamin B12 levels in a dose-dependent manner. Since vitamin B12 is essential to nutrition, metformin-induced B12 reduction may have detrimental effects in patients with T2DM and PCOS. Patients treated with metformin may benefit from vitamin B12 supplements. Because few clinical studies have assessed this directly, additional, well-designed trials are needed to confirm our findings.


We acknowledge the epidemiologists and clinical researchers and their work, observational studies and clinical trials, we cited and not cited, giving the indications and evidences for our systematic review. We also appreciate valuable advice from Professor Haoming Tian, Department of Endocrinology and Metabolism, West China Hospital of Sichuan University.

Author Contributions

Conceived and designed the experiments: JL. Performed the experiments: QL SL HQ. Analyzed the data: QL SL HQ. Wrote the paper: QL SL.


  1. 1. La Marca A, Artensio AC, Stabile G, Volpe A (2005) Metformin treatment of PCOS during adolescence and the reproductive period. Eur J Obstet Gynecol Reprod Biol 121: 3–7.
  2. 2. UK Prospective Diabetes Study (UKPDS) Group (1998) Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). Lancet 352: 854–865.
  3. 3. Filioussi K, Bonovas S, Katsaros T (2003) Should we screen diabetic patients using biguanides for megaloblastic anaemia? Aust Fam Physician 32: 383–384.
  4. 4. Ting RZ, Szeto CC, Chan MH, Ma KK, Chow KM (2006) Risk factors of vitamin B(12) deficiency in patients receiving metformin. Arch Intern Med 166: 1975–1979.
  5. 5. Wile DJ, Toth C (2010) Association of metformin, elevated homocysteine, and methylmalonic acid levels and clinically worsened diabetic peripheral neuropathy. Diabetes Care 33: 156–161.
  6. 6. Hoogeveen EK, Kostense PJ, Beks PJ, Mackaay AJ, Jakobs C, et al. (1998) Hyperhomocysteinemia Is Associated With an Increased Risk of Cardiovascular Disease, Especially in Non–Insulin-Dependent Diabetes Mellitus A Population-Based Study. Arterioscler Thromb Vasc Biol 18: 133–138.
  7. 7. Vrbíková J, Bičíková M, Tallová J, Hill M, Stárka L (2002) Homocysteine and steroids levels in metformin treated women with polycystic ovary syndrome. Exp Clin Endocrinol Diabetes 110: 74–76.
  8. 8. Higgins JPT, Green S (2011) Cochrane Handbook for Systematic Reviews of Interventions, version 5.1.0 [updated March 2011]. The Cochrane Collaboration. Available: Accessed 20 October 2013.
  9. 9. Moher D, Liberati A, Tetzlaff J, Altman DG (2009) Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. Ann Intern Med 151: 264–269.
  10. 10. Wells GA, Shea B, O'Connell D, Peterson P, Welch V, et al.. (2010) The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. Ottawa: Ottawa Hospital Research Institute. Available: Accessed 20 October 2013.
  11. 11. Wolever TMS, Assiff L, Basu T, Chiasson JL, Boctor M, et al. (2000) Miglitol, an alpha-glucosidase inhibitor, prevents the metformin-induced fall in serum folate and vitamin B12 in subjects with type 2 diabetes. Nutrition Research 20: 1447–1456.
  12. 12. Wulffele MG, Kooy A, Lehert P, Bets D, Ogterop JC, et al. (2003) Effects of short-term treatment with metformin on serum concentrations of homocysteine, folate and vitamin B12 in type 2 diabetes mellitus: a randomized, placebo-controlled trial. J Intern Med 54: 455–463.
  13. 13. Kilicdag EB, Bagis T, Zeyneloglu HB, Tarim E, Aalan E, et al. (2005) Homocysteine levels in women with polycystic ovary syndrome treated with metformin versus rosiglitazone: a randomized study. Hum Reprod 20: 894–899.
  14. 14. Carlsen SM, Kjotrod S, Vanky E, Romundstad P (2007) Homocysteine levels are unaffected by metformin treatment in both nonpregnant and pregnant women with polycystic ovary syndrome. Acta Obstet Gynecol Scand 86: 145–150.
  15. 15. Sahin M, Tutuncu NB, Ertugrul D, Tanaci N, Guvener ND (2007) Effects of metformin or rosiglitazone on serum concentrations of homocysteine, folate, and vitamin B12 in patients with type 2 diabetes mellitus. J Diabetes Complications 21: 118–123.
  16. 16. de Jager J, Kooy A, Lehert P, Wulffelé MG, van der Kolk J, et al. (2010) Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. Br Med J 340: c2181.
  17. 17. Kulambil Padinjakara RN, Miriyalaw A, Saravananw P (2009) Association of low vitamin B12 level and ischemic heart disease in diabetes patients. Diabetic Medicine 26: 46.
  18. 18. Toth C (2009) Metformin-associated upregulation in methylmalonic acid levels correlates with clinical deterioration in patients with diabetic peripheral neuropathy. Journal of the Peripheral Nervous System 14: 245–246.
  19. 19. Vischer UM, Ardigo S, Perrenoud L, Genet C, Registe-Rameau Y, et al. (2009) The high prevalence of malnutrition and unnecessary drug therapy in very old diabetic patients. Diabetologia 52: S352–S353.
  20. 20. Calvo C, Michael R, Plodkowski R (2010) The impact of metformin on vitamin B12 levels in subjects with type 2 diabetes. Journal of Investigative Medicine 58: 215.
  21. 21. Plodkowski RA, Chau DL, Michael RL, Calvo CM, Nguyen QT, et al. (2010) The impact of metformin on vitamin B12 levels in older people with type 2 diabetes. J Am Geriatr Soc 58: S185.
  22. 22. Prabhu M, Narmadha MP, Palanisamy A (2010) Assessment of plasma homocysteine, vitamin B12 and folic acid in type II diabetes patients treated with metformin. Indian J Pharm Sci 72: 280.
  23. 23. Gastaldi G, Mura T, Sultan A, Piot C, Cristol J-P, et al. (2011) Metformin, Vit B12 deficiency and silent myocardial ischaemia in type 2 diabetic patients. Diabetologia 54: S500–S501.
  24. 24. Radfar M, Jaberi doost M, Hadjibabaie M, Larijani B (2011) Vitamin B12 levels in type 2 diabetic patients receiving metformin. Int J Clin Pharm 33: 339.
  25. 25. Rajendran R, Brewster S, Partridge H (2011) Metformin, vitamin B12 and diabetes: A retrospective three-year survey of people attending the diabetic neuropathy clinic. Diabetic Medicine 28: 136.
  26. 26. Atwell C, Long A, Yoo W, Solomon S (2012) Metformin and proton pump inhibitors effect on vitamin B12 levels. Journal of Investigative Medicine 60: 435.
  27. 27. Farvid MS, Homayouni F (2012) Factors associated with vitamin B12, folate and homocysteine levels in persons with type 2 diabetes. Clinical Chemistry and Laboratory Medicine 50: A48.
  28. 28. Nag S, Bilous M, Winship S, Bilous R, Jones S (2012) High prevalence of vitamin B12 deficiency in patients on metformin based treatment: Data from the Teesside Anaemia in Diabetes Study. Diabetic Medicine 29: 173.
  29. 29. Narayanan RP, Anderson SG, Onyekwelu E, Heald AH (2012) Vitamin B12 is lower in metformin treated patients but haemoglobin is unaffected. Diabetic Medicine 29: 72.
  30. 30. Nurpeisov V (2012) Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency. J Am Geriatr Soc 60: S36.
  31. 31. Morrison CL, Morrison G, Purewal TS, Weston PJ (2013) Anaemia in a primary care population with diabetes. Diabetic Medicine 30: 22.
  32. 32. Tomkin GH, Hadden DR, Weaver JA, Montgomery DA (1971) Vitamin-B12 status of patients on long-term metformin therapy. Br Med J 2: 685–687.
  33. 33. Carpentier JL, Bury J, Luyckx A, Lefebvre P (1976) Vitamin B 12 and folic acid serum levels in diabetics under various therapeutic regimens. Diabete et Metabolisme 2: 187–190.
  34. 34. Adams JF, Clark JS, Ireland JT, Kesson CM, Watson WS (1983) Malabsorption of vitamin B12 and intrinsic factor secretion during biguanide therapy. Diabetologia 24: 16–18.
  35. 35. Hermann LS, Nilsson B, Wettre S (2004) Vitamin B12 status of patients treated with metformin: A cross-sectional cohort study. British Journal of Diabetes and Vascular Disease 4: 401–406.
  36. 36. Pongchaidecha M, Srikusalanukul V, Chattananon A, Tanjariyaporn S (2004) Effect of metformin on plasma homocysteine, vitamin B12 and folic acid: a cross-sectional study in patients with type 2 diabetes mellitus. Journal of the Medical Association of Thailand 87: 780–787.
  37. 37. Pflipsen MC, Oh RC, Saguil A, Seehusen DA, Topolski R (2009) The prevalence of vitamin B12 deficiency in patients with type 2 diabetes: A cross-sectional study. Journal of the American Board of Family Medicine 22: 528–534.
  38. 38. Tal S, Shavit Y, Stern F, Malnick S (2010) Association between vitamin B12 levels and mortality in hospitalized older adults. J Am Geriatr Soc 58: 523–526.
  39. 39. Nervo M, Lubini A, Raimundo FV, Faulhaber GAM, Leite C, et al. (2011) Vitamin B12 in metformin-treated diabetic patients: a cross-sectional study in Brazil. Revista Da Associacao Medica Brasileira 57: 46–49.
  40. 40. Chen S, Lansdown AJ, Moat SJ, Ellis R, Goringe A, et al. (2012) An observational study of the effect of metformin on B12 status and peripheral neuropathy. British Journal of Diabetes and Vascular Disease 12: 189–193.
  41. 41. Reinstatler L, Qi YP, Williamson RS, Garn JV, Oakley Jr GP (2012) Association of biochemical B12 deficiency with metformin therapy and vitamin B12 supplements: The National Health and Nutrition Examination Survey, 1999–2006. Diabetes Care 35: 327–333.
  42. 42. de Groot-Kamphuis DM, van Dijk PR, Groenier KH, Houweling ST, Bilo HJG, et al. (2013) Vitamin B12 deficiency and the lack of its consequences in type 2 diabetes patients using metformin. Netherlands Journal of Medicine 71: 386–390.
  43. 43. Obeid R, Jung J, Falk J, Herrmann W, Geisel J, et al. (2013) Serum vitamin B12 not reflecting vitamin B12 status in patients with type 2 diabetes. Biochimie 95: 1056–1061.
  44. 44. Sato Y, Ouchi K, Funase Y, Yamauchi K, Aizawa T (2013) Relationship between metformin use, vitamin B12 deficiency, hyperhomocysteinemia and vascular complications in patients with type 2 diabetes. Endocr J 60: 1275–1280.
  45. 45. Tomkin GH (1973) Malabsorption of vitamin B12 in diabetic patients treated with phenformin: a comparison with metformin. Br Med J 3: 673–675.
  46. 46. Greibe E, Trolle B, Bor MV, Lauszus FF, Nexo E (2013) Metformin lowers serum cobalamin without changing other markers of cobalamin status: A study on women with polycystic ovary syndrome. Nutrients 5: 2475–2482.
  47. 47. Kos E, Liszek MJ, Emanuele MA, Durazo-Arvizu R, Camacho P (2012) Effect of metformin therapy on vitamin D and vitamin B12 levels in patients with type 2 diabetes mellitus. Endocrine Practice 18: 179–184.
  48. 48. Caspary W, Creutzfeldt W (1971) Analysis of the inhibitory effect of biguanides on glucose absorption: inhibition of active sugar transport. Diabtologia 7: 379–385.
  49. 49. Caspary W, Zavada I, Reimold W, Deuticke U, Emrich D, et al. (1977) Alteration of bile acid metabolism and vitamin-B12-absorption in diabetics on biguanides. Diabtologia 13: 187–193.
  50. 50. Bauman WA, Shaw S, Jayatilleke E, Spungen AM, Herbert V (2000) Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care 23: 1227–1231.
  51. 51. Buvat DR (2004) Use of metformin is a cause of vitamin B12 deficiency. Am Fam Physician 69: 264–266.
  52. 52. Moore EM, Mander AG, Ames D, Kotowicz MA, Carne RP, et al. (2013) Increased risk of cognitive impairment in patients with diabetes is associated with metformin. Diabetes Care 36: 2981–2987.
  53. 53. Leung S, Mattman A, Snyder F, Kassam R, Meneilly G, et al. (2010) Metformin induces reductions in plasma cobalamin and haptocorrin bound cobalamin levels in elderly diabetic patients. Clin Biochem 43: 759–760.
  54. 54. Liu KW, Dai LK, Jean W (2006) Metformin-related vitamin B12 deficiency. Age Ageing 35: 200–201.
  55. 55. Callaghan TS, Hadden DR, Tomkin GH (1980) Megaloblastic anaemia due to vitamin B12 malabsorption associated with long-term metformin treatment. Br Med J 280: 1214–1215.