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Riboflavin side effects: is vitamin B2 safe?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Riboflavin has not been shown to cause harm, and no upper limit is set. The most common effect is bright yellow urine, which is expected. In a trial of 400 mg a day for three months, two minor effects were reported with riboflavin, loose stools and passing more urine, and none was serious.1,2,3

Key facts

  • No adverse effects have been reported from 400 mg a day taken for at least three months.2
  • Food intakes many times the recommended amount have shown no toxicity.2
  • NIH says riboflavin is not known to have clinically relevant interactions with medicines.2
  • Experts still urge caution with very large amounts, because data on high intakes are limited.2

Bright yellow urine

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.2

Riboflavin is a yellow pigment that glows under ultraviolet light. Because the body keeps only small amounts, the extra riboflavin from a multivitamin or B-complex is filtered into urine and colors it.2,4

High doses

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.2,4

In a three-month trial of 400 mg a day in 55 adults, two minor side effects were reported with riboflavin, diarrhea and passing more urine, against one with placebo. None was serious.3

Doses that high are used for migraine prevention under medical guidance, and medical experts consider riboflavin's side effects minimal. Use them only with your doctor's advice.1,2

No upper limit has been set for riboflavin, because no adverse effects from high intakes have been reported. The gut absorbs at most about 27 mg at a time, and the excess leaves in urine.2,4

Why high intakes seem harmless

Two things limit how much reaches the body. Riboflavin does not dissolve well in large amounts, and the gut absorbs only about 27 mg from a single dose. What is absorbed beyond the body's needs is excreted, mostly in urine.2,5

The Food and Nutrition Board, which sets US intake values, could not find adverse effects to base an upper limit on. It points out that a lack of reported harm is not the same as proof that very high intakes are harmless.2

Medicines and who should ask first

NIH lists no medicine that interacts with riboflavin in a clinically relevant way. Tell your doctor and pharmacist about it anyway, as with every supplement you take.1,2

  • Ask first if you are pregnant or breastfeeding and want more than a prenatal vitamin provides
  • Ask first before high doses for a health problem such as migraine1
  • Ask first if you have riboflavin transporter deficiency or another condition that affects absorption, because high-dose treatment is then a medical decision2

Questions people ask

Can riboflavin cause diarrhea?

Rarely, and only reported at high doses. In a three-month trial of 400 mg a day, two minor effects were reported with riboflavin, diarrhea and passing more urine. At the amounts in food and multivitamins, NIH reports no harm.1,3

Is riboflavin safe to eat in food?

Yes. Food intakes many times the recommended amount have shown no toxicity, possibly because the gut absorbs riboflavin only in limited amounts. It is also added to many fortified cereals and breads.2

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Sources

  1. Office of Dietary Supplements, National Institutes of Health. Riboflavin: Fact Sheet for Consumers. Updated May 11, 2022. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Riboflavin: Fact Sheet for Health Professionals. Updated May 11, 2022. Opened 2026-10-07.
  3. 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.
  4. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Dietary Reference Values for riboflavin. EFSA J. 2017;15(8):e04919. Opened 2026-10-07.
  5. Zempleni J, Galloway JR, McCormick DB. Pharmacokinetics of orally and intravenously administered riboflavin in healthy humans. Am J Clin Nutr. 1996;63(1):54-66. Opened 2026-10-07.

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