Quick answer
In a randomized trial of 400 mg of riboflavin a day for 3 months in 55 people, two minor side effects occurred on riboflavin, diarrhea and passing more urine, and one on placebo, abdominal cramps. None was serious. NIH says riboflavin has not been shown to cause any harm. Ask your doctor if diarrhea lasts.1,2
Key facts
- In a 3-month trial of 400 mg of riboflavin a day in 55 people, diarrhea and passing more urine were each reported once on riboflavin, and abdominal cramps once on placebo.1
- The trial's authors called all three events minor, and none was serious.1
- NIH says riboflavin has not been shown to cause any harm, and no upper limit has been set.2,3
- The effect NIH names is bright yellow urine, which it calls a minimal side effect.3
- The gut absorbs at most about 27 mg of riboflavin from one dose, and the excess leaves in urine.3
What did the riboflavin trial find about diarrhea?
The best evidence on riboflavin side effects comes from a 1998 randomized trial in 55 people with migraine. They took 400 mg of riboflavin or a placebo every day for 3 months. The authors reported three minor adverse events in total: diarrhea and passing more urine in the riboflavin group, and abdominal cramps in the placebo group. None was serious.1
| Group | Events reported | Serious |
|---|---|---|
| Riboflavin, 400 mg a day | Diarrhea; passing more urine | No |
| Placebo | Abdominal cramps | No |
With one person each, the trial cannot say how often riboflavin causes diarrhea. It shows only that stomach and bowel complaints were rare at this amount, and that they occurred on placebo as well.1
What does NIH say about riboflavin and side effects?
NIH's consumer sheet says riboflavin has not been shown to cause any harm. The professional sheet says no upper limit was set because no adverse effects from high intakes, including 400 mg a day for at least 3 months, have been reported, while adding that the Food and Nutrition Board urges caution about excessive amounts.2,3
Urine often turns bright yellow, because riboflavin is yellow and the body passes the extra in urine. Medical experts describe this discoloration as a minimal side effect.3
No adverse effects have been reported from high riboflavin intakes from food or supplements, including 400 mg a day for at least three months, though experts urge caution with very large amounts.3,4
What did the trial in children report?
A 2008 trial gave 48 children 200 mg of riboflavin a day or a placebo for migraine. It measured adverse effects among its secondary outcomes and reported no differences between riboflavin and placebo for the primary or secondary outcomes. The abstract does not list individual side effects, so it says little about diarrhea beyond that.5
What do the sources not say?
The sources checked do not report how often riboflavin at the 1.3 mg or so in a multivitamin causes digestive complaints, and they do not cover nausea, gas, constipation or headache as named effects. Many riboflavin supplements are sold in B complex products, whose other vitamins may have effects of their own. That is not a finding either way.1,3
Who should ask a doctor about riboflavin and diarrhea?
- Anyone whose diarrhea lasts, gets worse or comes with other symptoms.1
- Anyone taking a high dose such as the 400 mg a day used in the migraine trial, which is a medical use.1
- Anyone with a condition that affects how riboflavin is absorbed or used, such as riboflavin transporter deficiency or low thyroid hormone.3
- Anyone who takes prescription medicines and wants to add a riboflavin or B complex supplement.2
Questions people ask
Can riboflavin cause diarrhea?
In a randomized trial of 400 mg a day for 3 months in 55 people, one person on riboflavin had diarrhea, and one on placebo had abdominal cramps. None was serious. Ask your doctor if it lasts.1
Does riboflavin cause nausea?
None of the sources checked lists nausea as an effect of riboflavin, and NIH says riboflavin has not been shown to cause any harm. Ask your doctor if you have nausea after starting a supplement.2,3
See what we make
Every product shows its whole label and what its lab report covers. The store opens soon.
Sources
- Schoenen J, Jacquy J, Lenaerts M. Effectiveness of high-dose riboflavin in migraine prophylaxis. A randomized controlled trial. Neurology. 1998;50(2):466-470. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Riboflavin: Fact Sheet for Consumers. Updated May 11, 2022. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Riboflavin: Fact Sheet for Health Professionals. Updated May 11, 2022. Opened 2026-10-07.
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Dietary Reference Values for riboflavin. EFSA J. 2017;15(8):e04919. Opened 2026-10-07.
- MacLennan SC, Wade FM, Forrest KM, Ratanayake PD, Fagan E, Antony J. High-dose riboflavin for migraine prophylaxis in children: a double-blind, randomized, placebo-controlled trial. J Child Neurol. 2008;23(11):1300-1304. Opened 2026-10-11.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.