Does vitamin B12 cause constipation?

Probably not, although no trial was built to answer the question. A 2023 meta-analysis of supplements for chronic constipation found 8 randomized trials in 787 adults, and none of them tested vitamin B12. What exists instead is side-effect data from B12 trials. In a year-long trial in 600 Nepalese infants given 2 mcg a day, a dose close to everyday needs, constipation turned up in 1 child of 300 on B12 and 1 of 300 on placebo. The FDA label for B12 injections lists mild transient diarrhea as its only gut side effect. Constipation is not on it.

Unsettled. The answer rests on the absence of a signal. No trial we found measured bowel habit as an outcome on high doses of B12, and side-effect lists in small trials can miss a common, mild complaint. What the data do show is that constipation does not stand out in any B12 trial, label or review we found.

People who reported constipation, in the two B12 trials that counted it

The infant counts come from the trial's results on ClinicalTrials.gov, where side effects were not asked about systematically. The crossover trial was small and open-label, and no side effect of any kind was reported on B12. Counts only, so no intervals are drawn. Sources: cards ev-b12c-05 and ev-b12c-04 below.

Established. Iron does cause constipation. A Cochrane review found that daily iron supplements doubled hard stools or constipation in menstruating women, relative risk 2.07 (95% CI 1.35 to 3.17), across 8 studies with 1,036 women, on high quality evidence. Where B12 comes in a combined pill that also contains iron, the iron could explain constipation blamed on B12. That is our inference, and no study has tested it.

The supplement with trial evidence of causing constipation, as a relative risk

Randomized trials

Iron, not B12. High quality evidence from a Cochrane review. Our card gives no baseline rate, so the relative risk cannot be turned into a count of people. Source: card ev-b12c-07 below.

What the trials found

No trial asked the question

The 2023 meta-analysis looked for every randomized trial of a food, vitamin or mineral supplement in adults with chronic constipation. It found trials of kiwifruit, senna, magnesium oxide and two plant extracts, and none of B12. Only magnesium oxide clearly helped. People on it were 3.32 times as likely to respond (95% CI 1.59 to 6.92) and had 3.72 more bowel movements a week (95% CI 1.41 to 6.03). That is context for this page. The finding that matters here is the missing B12 trial.

Side effects in B12 trials

A Cochrane review of 3 randomized trials in 153 adults with B12 deficiency, given 1000 to 2000 mcg a day by mouth or injections for 3 to 4 months, found few adverse events. One trial reported none linked to treatment. In another, 2 of 30 people on oral B12 left early because of adverse events, and the review does not say what they were. The evidence on harms was rated very low quality, and constipation was not an outcome.

A meta-analysis of 15 randomized trials in 1,707 people with nerve damage from diabetes or shingles found no serious adverse events tied to mecobalamin, an active form of B12. Most of those trials were at high risk of bias, and minor complaints such as changes in bowel habit were not pooled in the abstract.

Two trials counted constipation directly. In a crossover trial of 26 Japanese men, each took the drug amoxapine and then B12 at 500 mcg three times a day, 4 weeks each. Two of 25 reported constipation on amoxapine and nobody reported any side effect on B12, in a small open-label trial where people reported their own symptoms. In the double-blind Nepal trial of 600 infants aged 6 to 11 months, the authors found no adverse effects of B12, and the registry results show 1 case of constipation in each arm of 300.

B12 from food

In US survey data on 13,885 adults, B12 intake was not significantly linked to constipation after adjusting for age, sex and ethnicity (p for trend 0.26). Folate was, with the highest intake quarter having lower odds of constipation, odds ratio 0.61 (95% CI 0.48 to 0.79). This was one 24-hour diet recall per person, and B12 in food comes with meat, fish, eggs and dairy, so it says nothing about tablets.

The other direction

Constipation may matter for B12 status more than B12 matters for constipation, though the evidence is thin. In a 12-week randomized trial of 30 frail nursing home residents aged 57 to 98 who used laxatives, 5.2 g of oat bran a day cut laxative use, and plasma B12 fell faster in the group without it (p below 0.05). In 524 Swedes aged 82 and over, laxative users had higher homocysteine and lower folate, while constipated people not on laxatives showed no such pattern. That abstract reports no B12 difference. A systematic review of B12 and gut bacteria found 22 studies, with no randomized trials in humans and no bowel habit outcomes.

Who should be careful

The clearest caution with B12 runs the other way, toward running short. The NIH notes that some medicines, including metformin and stomach acid blockers, may affect B12 levels. B12 itself has little known harm. The NIH sets no upper intake limit because its potential for toxicity is low, which means harm was not found and does not make high doses useful. A 2026 narrative review found B12 supplements generally well tolerated, with rare acne-like and allergic reactions, and it does not mention constipation. The children's evidence here is the Nepal trial at a small daily dose. We have no card on B12 and bowel habit in pregnancy, and that gap is not a sign of safety.

Not for everyone. If you take metformin or an acid blocker long term, these medicines may affect B12 levels, and it is worth asking your doctor whether to check them. If constipation began with a new combination supplement, check the label for iron, the ingredient with trial evidence of causing it.

What expert bodies say

The FDA-approved label for cyanocobalamin injection lists mild transient diarrhea as the only gastrointestinal adverse reaction, and the label for the nasal spray has the same entry. The NIH Office of Dietary Supplements fact sheet for health professionals sets no upper limit and lists the medicines above. We found no expert body that names constipation as a side effect of B12.

How we searched

Searched: a local copy of PubMed, queried on 6 October 2026 for vitamin B12, cobalamin, cyanocobalamin or methylcobalamin with constipation, bowel, gastrointestinal effects, diarrhea or adverse events (837 hits across all study types, 15 naming constipation). Further queries covered vitamins and constipation in general (59 hits), gut symptoms in B12 deficiency (115, mostly case reports), iron and constipation reviews (8) and the Nepal infant trial (10). We also searched FDA adverse event reports for B12 products, where constipation was not among the ten most reported problems, though such reports are not rates. Other sources were the NIH Office of Dietary Supplements fact sheet, drug label interactions, ClinicalTrials.gov (no trial of B12 and constipation), DailyMed, and Retraction Watch for every journal source used. None was retracted.

Included: a meta-analysis of supplements for constipation, a Cochrane review of oral against injected B12, a meta-analysis of mecobalamin and a Cochrane review of iron. Two randomized trials that counted constipation, a trial of oat bran in laxative users, US survey data, a Swedish cohort, a review of B12 and gut bacteria, a 2026 narrative review, the FDA label and two NIH passages, cited below.

Excluded: a 2026 Cochrane review of B12 in children, which found no trial reporting adverse effects and repeats the Nepal point. A trial in Angelman syndrome that gave B12 with three other ingredients, so constipation there cannot be put down to B12. Case reports of gut symptoms in deficiency, and surveys of diet in autism and bowel disease.

What we read: abstracts for most sources, with each quotation checked against the abstract text. Full text for the Nepal trial and the US survey analysis, and the side-effect results of the Nepal trial on ClinicalTrials.gov. The FDA label was read in full from DailyMed.

What we could not get: the NIH consumer fact sheet on B12, which refused our requests. It is said to list constipation as a symptom of deficiency, a separate question from supplements, and we could not confirm the wording, so it has no card. The Nepal trial's side-effect table by type is not in the free full text, and the registry results stand in for it.

What would change this answer

A randomized trial, or a pooled analysis of trial side effects, that records bowel habit on oral B12 at 1000 mcg a day and above. We found none, in print or registered.
A trial comparing a multivitamin with and without iron on constipation, which would test our inference about where the complaint comes from.
Checked wording from the NIH consumer fact sheet on constipation in B12 deficiency, which would answer the reverse question.

How much B12 you need, who runs short and where it comes from in food is in the vitamin B12 guide.

The rest of the nutrient, in one place. Vitamin B12: who actually needs to worry, and why the damage can be permanent → What it does, how much you need, who runs short, and what too much does, with the evidence level shown on every line.

The full guide

Vitamin B12: who actually needs to worry, and why the damage can be permanent

The same question for other foods (constipation)

Sources

ev-b12c-01 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 787
Eight RCTs (787 participants) were included, investigating kiwifruit (n = 3 RCTs), senna (n = 2), magnesium oxide (n = 2), Ziziphus jujuba (n = 1), and Malva Sylvestris (n = 1) supplements.
Who: adults with chronic constipation in RCTs of food, vitamin or mineral supplements
Effect: no B12 trials found; magnesium oxide responders RR 3.32 (95% CI 1.59 to 6.92), +3.72 bowel movements/week (1.41 to 6.03)
Certainty: Absence of B12 trials is the finding for this pair; Mg numbers are context.
Neurogastroenterol Motil, 2023 · checked 2026-10-06 · we read the abstract
ev-b12c-02 · Meta-analysis or systematic review · Cochrane systematic review of randomized controlled trials, no meta-analysis · n = 153
One trial reported that 2 of 30 participants (6.7%) in the oral vitamin B12 group left the trial early due to adverse events.
Who: adults with vitamin B12 deficiency, mean age 38.6 to 72 years, oral 1000 to 2000 ug/day or IM, 3 to 4 months
Effect: 2 of 30 (6.7%) left early due to adverse events on oral B12 in one trial; no treatment-related adverse events in another; type of events not named
Certainty: Very low quality evidence for adverse events (GRADE); constipation not reported as an outcome.
Cochrane Database Syst Rev, 2018 · checked 2026-10-06 · we read the abstract
ev-b12c-03 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 1,707
No serious adverse events associated with mecobalamin were reported during the treatment periods.
Who: patients with diabetic peripheral neuropathy or herpetic neuropathy
Effect: no serious adverse events associated with mecobalamin
Certainty: 73% of trials at high risk of bias; minor events such as bowel changes not pooled in the abstract.
J Altern Complement Med, 2020 · checked 2026-10-06 · we read the abstract
ev-b12c-04 · Randomized controlled trial(s) · open-label randomized crossover trial · n = 26
One patient (4%) reported sleepiness and 2 (8%) reported constipation while receiving amoxapine. No adverse events were reported during vitamin B12 treatment.
Who: men with retrograde ejaculation, Japan; amoxapine 50 mg/day vs vitamin B12 500 ug three times daily, 4 weeks each
Effect: constipation 2 of 25 (8%) on amoxapine; no adverse events on vitamin B12
Certainty: Small, open-label, adverse events self-reported; only 4 weeks of B12.
Int Braz J Urol, 2017 · checked 2026-10-06 · we read the abstract
ev-b12c-05 · Randomized controlled trial(s) · double-blind placebo-controlled randomized trial · n = 600
We found no adverse effects of vitamin B12 supplementation (S3 Table).
Who: marginally stunted infants aged 6 to 11 months, Bhaktapur, Nepal; 2 ug B12 daily for 12 months
Effect: no adverse effects of B12; registry results: constipation 1 of 300 (B12) vs 1 of 300 (placebo)
Certainty: Constipation counts from ClinicalTrials.gov NCT02272842 results (other adverse events, non-systematic assessment); dose near the RDA, not high-dose.
PLoS Med, 2020 · checked 2026-10-06 · we read the fulltext
ev-b12c-06 · Observational data · cross-sectional analysis of a national survey · n = 13,885
However, no significant relationship was detected between vitamin B12 intake and constipation in Model 2 (p for trend = 0.26).
Who: US adults aged 20 and over, NHANES 2005 to 2010, constipation by Bristol Stool Form Scale, one 24-h recall
Effect: B12 intake vs constipation p for trend 0.26; folate Q4 vs Q1 OR 0.61 (95% CI 0.48 to 0.79)
Certainty: Single 24-h recall; food B12 travels with protein foods, not with supplements.
Frontiers in Nutrition, 2025, Evaluating the link between the dietary intake of vitamin B and constipation: a population-based study · checked 2026-10-06 · we read the fulltext
ev-b12c-07 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized controlled trials · n = 1,036
eight studies recruiting 1036 women identified an increased prevalence of hard stools/constipation (RR 2.07, 95% CI 1.35 to 3.17, high quality evidence).
Who: menstruating women in trials of daily oral iron supplementation
Effect: hard stools or constipation RR 2.07 (95% CI 1.35 to 3.17), 8 studies, high quality evidence
Certainty: About iron, not B12; the link to multivitamin users is an inference, not tested.
Cochrane Database Syst Rev, 2016 · checked 2026-10-06 · we read the abstract
ev-b12c-08 · Randomized controlled trial(s) · randomized controlled parallel trial · n = 30
In the control group, plasma B12 decreased faster (p < 0.05).
Who: frail nursing home residents aged 57 to 98 on laxative therapy; 5.2 g/day oat bran vs habitual diet, 84 days
Effect: laxative use decreased with oat bran (p < 0.001); plasma B12 decreased faster in controls (p < 0.05)
Certainty: Very small; tests the reverse direction (constipation care and B12 status), not B12 causing constipation.
J Nutr Health Aging, 2010 · checked 2026-10-06 · we read the abstract
ev-b12c-09 · Observational data · population-based cohort, cross-sectional biochemistry · n = 524
Homocysteine levels were increased and those of folate reduced in aged subjects on laxatives.
Who: population-based sample of adults aged 82 years or older, Sweden
Effect: laxative use: homocysteine raised, folate lowered; constipated people without laxatives: no significant effect
Certainty: Abstract does not report a B12 difference; frailty adjusted.
Eur J Clin Pharmacol, 2004 · checked 2026-10-06 · we read the abstract
ev-b12c-10 · Position of an expert body · narrative review · n = —
B12 supplementation is generally well tolerated.
Who: people taking vitamin B12 supplements, oral 0.02 to 1 mg/day or IM 1 to 5 mg/week
Effect: generally well tolerated; rare acneiform and hypersensitivity reports; constipation not mentioned
Certainty: Narrative review without a pooled analysis.
Nutrients, 2026 · checked 2026-10-06 · we read the abstract
ev-b12c-11 · Position of an expert body · drug label · n = —
Mild transient diarrhea.
Who: patients receiving parenteral vitamin B12 (cyanocobalamin injection, USP)
Effect: GI adverse reaction on label: mild transient diarrhea; constipation absent
Certainty: Two requests: curl of the DailyMed SPL XML and WebFetch of the DailyMed label page returned the same wording; the Nascobal nasal spray label has the same entry.
DailyMed, Cyanocobalamin Injection, USP label, Fresenius Kabi USA, revised 2024-12-12 · checked 2026-10-06 · we read the section
ev-b12c-12 · 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, NIH Office of Dietary Supplements vitamin B12 fact sheet
Effect: no Tolerable Upper Intake Level set by the Food and Nutrition Board
Certainty: No UL does not mean high doses are useful; it means harm was not found.
NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-06 · we read the section
ev-b12c-13 · Position of an expert body · agency fact sheet · n = —
Vitamin B12 supplements may interact with medications, and some medications, including gastric acid inhibitors and metformin, may affect vitamin B12 levels.
Who: people taking gastric acid inhibitors or metformin regularly
Effect: gastric acid inhibitors and metformin may affect B12 levels
Certainty: Agency summary.
NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-06 · we read the section
ev-b12c-14 · Observational data · systematic review of in vitro, animal and observational studies · n = 22 studies
To date, few prospective observational studies and no randomized trials have been conducted to examine the effects of vitamin B-12 on the human gut microbiome.
Who: in vitro, animal and human observational studies of B12 and gut microbiome
Effect: no randomized trials; human findings heterogeneous
Certainty: Mixed designs; no bowel habit outcomes.
Adv Nutr, 2022 · checked 2026-10-06 · we read the abstract

How this page was made. Evidence was extracted from primary sources into cards, every number was re-checked against the source, and the text was written from those cards. Bioma Learn has no human medical reviewer at this time, and we say so rather than invent one. Methodology. This is information, not medical advice.