Does vitamin B12 help with weight loss?

No. The largest and longest randomized test, the B-PROOF trial, gave 2,919 adults aged 65 and over 500 micrograms of B12 with folic acid every day for 2 years, and found no significant effect on BMI, fat mass or lean mass against placebo. We found no trial and no systematic review that tests B12 alone, as a pill or an injection, for weight loss. For the B12 shots sold at weight-loss clinics, we found no trial at all.

Myth. B12 is sold as a weight-loss aid and an energy booster. In older adults with high homocysteine, two years of B12 with folic acid did not change BMI or body fat. The trial authors wrote that the link between B12 status and body composition seen in observational data was not confirmed when the vitamins were randomly assigned.

What the trials found

B12 with folic acid, two years

B-PROOF is the strongest evidence on this question and it is still indirect. It combined B12 with folic acid, so the effect of B12 alone cannot be separated. Everyone in it was 65 or older and had raised homocysteine. Within those limits the answer was clear: no significant change in BMI, fat mass index or fat-free mass index over 2 years.

B vitamins in people with overweight or obesity

A 2026 network meta-analysis of supplement trials in children and adults with overweight or obesity found that vitamin B did not significantly change BMI against placebo (mean difference -0.20 kg/m2, 95% CI -1.21 to 0.82). Waist size did not change significantly either (-0.68 in (95% CI -1.5 to 0.17; metric: -1.72 cm, -3.89 to 0.44)). That analysis treated vitamin B as one class and also accepted non-randomized trials, which is why we rate it below a trial.

What B12 supplements reliably do

Across 21 randomized trials, B12 supplements lowered blood homocysteine by 4.15 umol/L (95% CI 3.45 to 4.86) against control, more so with 12 weeks or more and doses above 500 micrograms a day. That is a blood marker, not a weight outcome. The energy argument fares no better. A meta-analysis of randomized trials in people without B12 deficiency found only one trial on unexplained fatigue, so fatigue could not be pooled at all.

Why the idea exists

A review of 19 observational studies with 7,055 people found serum B12 lower in people with obesity than in controls, by 56 pmol/L (95% CI 23 to 90). Every study in it was at high risk of bias, and the reviewers said they could not establish an inverse association between B12 and BMI. In children and teenagers the difference was not there: a 2025 meta-analysis of 20 studies with 7,791 young people found no significant difference in B12 between those with and without obesity (SMD -0.24, 95% CI -0.53 to 0.06). A lower blood level in a group is a different thing from a vitamin that causes weight loss, and the one long trial that tested it did not find one.

Who should be careful

The real link between B12 and weight runs the other way. Two common weight treatments lower B12.

Not for everyone. Metformin lowers B12. In a meta-analysis of 4 trials in people with type 2 diabetes, it cut serum B12 by 57 pmol/L (95% CI 35 to 79) within 6 weeks to 3 months. Across 31 studies of people with diabetes, those on metformin had about twice the risk of B12 deficiency (RR 2.09, 95% CI 1.49 to 2.93), and these studies mixed designs. In the Diabetes Prevention Program, adults with overweight or obesity on metformin had low or borderline B12 at 5 years in 19.1 percent against 9.5 percent on placebo.

Weight-loss surgery is the second group. Operations on the stomach and gut can remove the cells that the body needs to absorb B12, according to the US Office of Dietary Supplements. A meta-analysis of 54 studies that followed patients for 5 to 17 years after surgery found B12 deficiency in 8.5 percent. If you take metformin or have had bariatric surgery, B12 is a level to have checked, and any supplement is a matter of correcting a shortfall with the person treating you.

No upper intake limit has been set for B12 because its potential for toxicity is low. That is a statement about toxicity, not about benefit: it says nothing about whether B12 helps weight.

What expert bodies say

The US Office of Dietary Supplements notes that B12 is promoted as an energy booster and states that supplements appear to have no benefit for performance without a deficiency. The European Union authorizes one B12 claim about energy: that it contributes to normal energy-yielding metabolism. The EU register holds no B12 claim about body weight. A claim about normal metabolism describes what the vitamin does in every body, and marketing that reads it as fat burning goes beyond what was authorized.

How we searched

What would change this answer

• A randomized trial of B12 alone, pill or injection, with weight as the main outcome, in adults of working age. We found none published and none in the trial registry we searched.
• A trial that recruits people with low B12 and measures weight. If B12 matters for weight anywhere, it would show there, and B-PROOF did not select for it.
• A trial of B12 alongside metformin or after bariatric surgery that reports weight as well as B12 levels.

Sources

1. ev-b12w-01 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial · n = 2919
   However, random allocation of B-vitamins did not have a significant effect on changes in BMI, FMI or FFMI during 2 years of intervention.
   
   Who: 2919 participants aged 65 years and over with elevated homocysteine
   Effect: no significant effect on changes in BMI, fat mass index or fat-free mass index over 2 years
   Certainty: The largest and longest randomized test of B12 on body composition; combined with folic acid, older adults only.
   Eur J Nutr, 2020 · checked 2026-10-07 · we read the abstract
2. ev-b12w-02 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial with cross-sectional analyses · n = 2919
   Although observational data suggested that folate and vitamin B12 status are associated with body composition, random allocation of a supplement with both B-vitamins combined versus placebo did not confirm an effect on BMI or body composition.
   
   Who: older adults with elevated homocysteine, folic acid 400 ug plus vitamin B12 500 ug vs placebo
   Effect: observational associations with BMI and fat mass not reproduced by randomized supplementation
   Certainty: Key to the question: association with weight does not mean B12 causes weight change.
   Eur J Nutr, 2020 · checked 2026-10-07 · we read the abstract
3. ev-b12w-03 · Observational data · systematic review and network meta-analysis of randomized and non-randomized trials · n = —
   In comparison, inulin (MD: −0.24 kg/m2; 95% CI: −1.06 to 0.57), butyrate (MD: −0.26 kg/m2; 95% CI: −1.25 to 0.73), vitamin B (MD: −0.20 kg/m2; 95% CI: −1.21 to 0.82), and LC n3-PUFA (MD: −0.18 kg/m2; 95% CI: −1.20 to 0.83) did not result in statistically significant changes when compared to placebo.
   
   Who: children or adults with overweight or obesity in randomized and non-randomized trials of nutraceuticals vs placebo
   Effect: BMI MD -0.20 kg/m2 (95% CI -1.21 to 0.82); waist MD -0.68 in (95% CI -1.5 to 0.17; metric: -1.72 cm, -3.89 to 0.44); both not significant
   Certainty: Level C because non-randomized trials were eligible. 'Vitamin B' is a class (modeled dose 14 mg/day), not B12 alone.
   Front Nutr, 2026 · checked 2026-10-07 · we read the fulltext
4. ev-b12w-04 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 6276
   Further, only one study reported effects on idiopathic fatigue, and therefore, no analysis was possible.
   
   Who: patients without advanced neurological disorders or overt vitamin B12 deficiency
   Effect: no effect of B12 alone or B complex on cognition or depressive symptoms; fatigue not poolable (1 trial)
   Certainty: Addresses the usual claim that B12 shots boost energy and so help weight loss; no weight outcome was studied.
   Nutrients, 2021 · checked 2026-10-07 · we read the abstract
5. ev-b12w-05 · Meta-analysis or systematic review · systematic review and meta-regression of randomized controlled trials · n = 1625
   Hcy levels were significantly lower after B12 supplementation compared with the control group (pooled weighted mean difference, -4.15 μmol/L; 95% confidence interval, -4.86, -3.45; P < 0.001), and this reduction was even greater with intervention durations ≥12 weeks and doses >500 µg/d.
   
   Who: participants of 21 RCTs of B12 supplementation vs control
   Effect: homocysteine WMD -4.15 umol/L (95% CI -4.86 to -3.45); larger with 12 weeks or more and doses above 500 ug/day
   Certainty: Shows what B12 supplements reliably change; nothing here links that to body weight.
   Nutr Rev, 2024 · checked 2026-10-07 · we read the abstract
6. ev-b12w-06 · Observational data · systematic review with network and pairwise meta-analysis of observational studies · n = 7055
   The pairwise results from this sub-network showed lower levels of B12 in people with higher body mass indices: obesity versus control difference in means (MD) -56 pmol L-1 (95% CI -90, -23), obesity versus overweight MD -21 pmol L-1 (95% CI -37, -5) and overweight versus control MD -51 pmol L-1 (95% CI -51, -24).
   
   Who: general and clinical populations compared across BMI groups, 19 observational studies
   Effect: obesity vs control MD -56 pmol/L (95% CI -90 to -23); overweight vs control MD -51 pmol/L; all studies high risk of bias
   Certainty: Association only; lower B12 may reflect diet, volume of distribution or drugs such as metformin, not a cause of weight gain.
   Obes Rev, 2018 · checked 2026-10-07 · we read the abstract
7. ev-b12w-07 · Observational data · systematic review with network and pairwise meta-analysis of observational studies · n = 7055
   This review did not establish an inverse association (or J-curve) between serum or plasma B12 concentrations and body mass index, but the direct pairwise evidence is consistent with an inverse association and supports further investigation.
   
   Who: 19 observational studies comparing B12 across BMI groups
   Effect: no established inverse association or J-curve; further investigation needed
   Obes Rev, 2018 · checked 2026-10-07 · we read the abstract
8. ev-b12w-08 · Observational data · systematic review and meta-analysis of observational studies · n = 7791
   There were no significant differences between children/adolescents with and without obesity with respect to serum vitamin B12 levels (SMD: -0.24; 95% CI: -0.53 to 0.06; p > 0.05, I2 = 74.93%) and folate levels (SMD: -0.12; 95% CI: -0.29 to 0.06; p > 0.05, I2 = 19.6%).
   
   Who: children and adolescents with and without obesity, 20 studies
   Effect: serum B12 SMD -0.24 (95% CI -0.53 to 0.06), not significant; homocysteine higher in obesity (SMD 0.77)
   Front Public Health, 2025 · checked 2026-10-07 · we read the abstract
9. ev-b12w-09 · Meta-analysis or systematic review · systematic review of 26 papers with meta-analysis of 4 trials · n = 4 trials
   Meta-analysis performed on four trials demonstrated a statistically significant overall mean B12 reducing effect of metformin of 57pmol/L [WMD (fixed)=-0.57 (95% CI: -35 to -79pmol/L)] after 6weeks to 3months of use.
   
   Who: patients with type 2 diabetes taking metformin; meta-analysis of 4 trials within a review of 26 papers
   Effect: mean B12 change -57 pmol/L (95% CI -35 to -79) after 6 weeks to 3 months
   Certainty: People losing weight on metformin are a group in whom B12 testing matters, not one in whom B12 helps weight.
   Diabetes Metab, 2016 · checked 2026-10-07 · we read the abstract
10. ev-b12w-10 · Observational data · meta-analysis of randomized and observational studies · n = 31 studies
    Compared with diabetic patients not taking metformin, patients taking metformin had a significantly higher risk of vitamin B12 deficiency (RR 2.09; 95% CI 1.49, 2.93; P < 0.0001; I2 = 64%) and significantly lower serum vitamin B12 concentrations (MD -63.70; 95% CI -74.35, -53.05] pM; P < 0.00001; I2 = 87%), which depended on dose and duration of treatment.
    
    Who: diabetic patients taking vs not taking metformin, 31 studies
    Effect: B12 deficiency RR 2.09 (95% CI 1.49 to 2.93); serum B12 MD -63.70 pmol/L; dose and duration dependent
    Certainty: Mixed designs (case-control included), so C.
    J Diabetes, 2019 · checked 2026-10-07 · we read the abstract
11. ev-b12w-11 · Randomized controlled trial(s) · secondary analysis of a randomized controlled trial and its open-label follow-up · n = 1715
    Combined low and borderline-low B12 (≤ 298 pg/mL) was more common in MET at 5 years (19.1 vs 9.5%; P < .01) and 13 years (20.3 vs 15.6%; P = .02).
    
    Who: adults with overweight/obesity and elevated glucose, metformin 850 mg twice daily vs placebo
    Effect: low or borderline-low B12: 19.1% vs 9.5% at 5 years, 20.3% vs 15.6% at 13 years; OR 1.13 per year of metformin use
    J Clin Endocrinol Metab, 2016 · checked 2026-10-07 · we read the abstract
12. ev-b12w-12 · Observational data · systematic review and meta-analysis of clinical studies (prevalence) · n = 54 studies
    The most prevalent vitamin deficiencies after surgery were vitamin D (35.8%), followed by vitamin E (16.5%), vitamin A (13.4%), vitamin K (9.6%), and vitamin B12 (8.5%).
    
    Who: patients after bariatric surgery with follow-up of 5 to 17 years, 54 articles
    Effect: long-term B12 deficiency prevalence 8.5%; higher after Roux-en-Y gastric bypass
    Langenbecks Arch Surg, 2024 · checked 2026-10-07 · we read the abstract
13. ev-b12w-13 · Position of an expert body · agency fact sheet · n = —
    However, vitamin B12 supplementation appears to have no beneficial effect on performance in the absence of a nutritional deficit [99,100].
    
    Who: people with sufficient vitamin B12 status
    Effect: no benefit on performance or endurance in the absence of a deficit
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
14. ev-b12w-14 · Position of an expert body · agency fact sheet · n = —
    Surgical procedures in the gastrointestinal tract, such as for weight loss or to remove all or part of the stomach, can cause a complete or partial loss of cells that secrete hydrochloric acid and cells that secrete intrinsic factor [54,55].
    
    Who: people who have had gastrointestinal surgery, including for weight loss
    Effect: complete or partial loss of acid-secreting and intrinsic-factor-secreting cells
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
15. ev-b12w-15 · Position of an expert body · agency fact sheet · n = —
    Metformin, an antihyperglycemic agent used as first-line treatment for prediabetes and diabetes, might reduce the absorption of vitamin B12 and significantly reduce serum vitamin B12 concentrations [103].
    
    Who: people taking metformin for prediabetes or diabetes
    Effect: reduced B12 absorption and serum concentrations
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
16. ev-b12w-16 · Position of an expert body · agency fact sheet · n = —
    The FNB did not establish a UL for vitamin B12 because of its low potential for toxicity [1].
    
    Who: general population
    Effect: no UL set by the Food and Nutrition Board
    Certainty: Low toxicity is not evidence of a weight benefit; injections still carry procedure costs.
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
17. ev-b12w-17 · Position of an expert body · EU health claims register entry · n = —
    POL-HC-6482 Vitamin B12 Vitamin B12 contributes to normal energy-yielding metabolism Authorised
    
    Who: EU food and supplement labeling
    Effect: authorized health claim on energy metabolism; no authorized B12 claim on weight
    Certainty: Energy-metabolism wording is often stretched in marketing to imply fat burning; the register supports no such claim.
    EU Register on nutrition and health claims, claim POL-HC-6482, snapshot 2026-09-21 · checked 2026-10-07 · we read the section