Quick answer

The label of a vitamin B12 injection lists mild transient diarrhea among its adverse reactions. NIH says B12 has not been shown to cause any harm, even at high doses, and a Cochrane review of 3 trials of oral versus injected B12 found very low-quality evidence on side effects. Ask your doctor if diarrhea lasts.^1,2,3

Key facts

The label of a vitamin B12 injection lists mild transient diarrhea under gastrointestinal adverse reactions.¹
NIH says vitamin B12 has not been shown to cause any harm, even at high doses.²
A Cochrane review of 3 trials with 153 people compared oral and injected B12 and rated its evidence on adverse events very low quality.³
In one of those trials, 2 of 30 people (6.7%) taking oral B12 left early because of adverse events, and another trial saw no treatment-related adverse events.³
A 2026 Cochrane review of oral B12 in children found little to no difference in adverse effects, though few trials reported them.⁴

What does the vitamin B12 injection label list?

The only official document checked that names diarrhea is the label of a prescription vitamin B12 injection. Under adverse reactions it lists, by body system, anaphylactic shock and death with injected B12, pulmonary edema, congestive heart failure and peripheral vascular thrombosis early in treatment, polycythemia vera, itching and a transitory rash, a feeling of swelling of the entire body, and, under gastrointestinal, mild transient diarrhea.¹

Adverse reactions listed on a vitamin B12 injection label¹
Body system	Reaction listed
Gastrointestinal	Mild transient diarrhea
Dermatological	Itching and transitory exanthema, a short-lived rash
Cardiovascular	Pulmonary edema, congestive heart failure and peripheral vascular thrombosis early in treatment
Hematological	Polycythemia vera
Generalized	Anaphylactic shock and death have been reported with injected B12

These reactions come from an injected product used under medical care for deficiency. The label does not say how often diarrhea happens and does not separate it from the effects of the underlying illness, so it cannot be read as a rate for oral supplements.¹

What does NIH say about B12 supplements and harm?

NIH's fact sheets are a different kind of source. The consumer sheet says vitamin B12 has not been shown to cause any harm, even at high doses, and the professional sheet says no upper limit was set because of low potential for toxicity. Neither names diarrhea, constipation, nausea or any other digestive effect as a known result of B12.^2,5

The absence is informative but not conclusive. The sheets summarize the research; they do not report counts of side effects from supplements taken by the public.^2,5

What do trials of oral B12 report?

Trial data are sparse. The 2018 Cochrane review of oral versus injected B12 for deficiency found 3 trials with 153 people. Two reported on adverse events with very low quality evidence. One said no treatment-related adverse events were seen in either group, and another said 2 of 30 people (6.7%) in the oral group left the trial early because of adverse events. The authors judged that oral B12 appeared as safe as injected B12, at very low certainty, and asked for better reporting of adverse events.³

The 2026 Cochrane review of children under 12 gives the same picture from another group. In the one trial that compared adverse effects between B12 with other nutrients and the same formula without B12, 9.2% of children versus 5.9% had one, which the review called little to no difference at low certainty. It added that few studies reported adverse effects at all.⁴

What do the sources not say about B12 and the gut?

The sources checked do not say how often oral B12 causes diarrhea, nausea or constipation, and none lists constipation as an effect. They also do not separate B12 from the other ingredients in combination products such as B complex pills and multivitamins. That is not a finding either way.^2,3,5

NIH does list stomach and bowel conditions, and stomach and weight-loss surgery, among the causes of B12 deficiency, so digestive symptoms and B12 questions can arise from the same underlying condition. A doctor can tell them apart.⁵

Who should ask a doctor about diarrhea and vitamin B12?

Anyone whose diarrhea lasts, gets worse or comes with other symptoms.¹
Anyone prescribed B12 injections or high-dose B12 for a deficiency, who should report any reaction to the prescriber.^1,3
Anyone taking metformin or a proton pump inhibitor, since these can lower B12 levels.⁵
Anyone with a stomach or bowel condition or past stomach surgery, where B12 absorption is already a question.⁵

Questions people ask

Does b12 cause diarrhea?

The label of a B12 injection lists mild transient diarrhea among adverse reactions, but NIH says B12 has not been shown to cause any harm even at high doses. How often oral B12 causes it is not reported. Ask your doctor if it lasts.^1,2

Does vitamin B12 cause constipation or diarrhea?

None of the sources checked lists constipation as an effect of B12. The injection label lists mild transient diarrhea. Ask your doctor if either lasts.^1,2

Does vitamin B12 cause nausea?

NIH names no nausea or other digestive effect from B12 in its fact sheets, and the injection label does not list nausea. A Cochrane review found very low-quality evidence on side effects of oral B12. Ask your doctor if you have persistent nausea.^1,2,3

Can too much B12 upset the stomach?

NIH says B12 has not been shown to cause any harm even at high doses and sets no upper limit. The sources checked do not report how often stomach upset occurs. Ask your doctor or pharmacist about any symptom after starting a supplement.^2,5

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This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.