What happens when you have too much B2?

December 16, 2025 •

4 min read

According to the National Institutes of Health, a Tolerable Upper Intake Level (UL) has not been established for riboflavin because no toxic level has been observed from food or long-term, high-dose supplements. The body's limited absorption and efficient excretion mechanisms make riboflavin toxicity exceptionally rare.

Quick Summary

Excessive intake of riboflavin, or vitamin B2, is generally considered safe, with the most common effect being a harmless, bright yellow discoloration of the urine. Very high supplemental doses may cause mild gastrointestinal upset, though the body flushes out most of the excess.

Key Points

Limited Absorption: The body can only absorb a certain amount of riboflavin from a single dose, typically around 27 mg, making overdose difficult.
Water-Solubility: As a water-soluble vitamin, any excess riboflavin is not stored by the body but is instead excreted in the urine.
Bright Yellow Urine: A harmless, common side effect of high riboflavin intake is bright yellow or orange urine, known as flavinuria.
Mild Gastrointestinal Issues: Very high supplemental doses of B2 may occasionally cause mild side effects such as nausea or diarrhea in some individuals.
Low Toxicity Risk: There is no established Tolerable Upper Intake Level for riboflavin because no adverse effects from high intakes have been systematically proven.
Potential Drug Interactions: Individuals taking certain medications should consult a doctor before starting a high-dose B2 supplement to avoid potential interactions.
Therapeutic Uses: High doses of riboflavin are sometimes medically prescribed for conditions like migraines and high homocysteine levels, demonstrating a high safety profile even at larger amounts.

Understanding Riboflavin's Water-Soluble Nature

Riboflavin, also known as vitamin B2, is a crucial water-soluble vitamin. This characteristic is key to understanding what happens when you have too much B2. Unlike fat-soluble vitamins (like A and D) that the body stores in fatty tissues, water-soluble vitamins are not stored in significant amounts. Any excess riboflavin that is not needed for cellular functions is simply filtered out by the kidneys and excreted in the urine. This built-in, natural process is the primary reason why overdose is highly unlikely in healthy individuals.

The Body's Limited Absorption Mechanism

Even when consuming very high doses, the small intestine has a limited capacity to absorb riboflavin at one time. Studies have shown that the maximum amount absorbed from a single oral dose is around 27 mg. This absorption saturation point further prevents potentially dangerous levels of the vitamin from entering the bloodstream, serving as a natural defense against toxicity. The unabsorbed portion continues through the digestive tract and is excreted in the feces.

Side Effects of High Riboflavin Intake

For most people, the side effects of too much B2 are benign and temporary. The most noticeable effect is a cosmetic change that is no cause for alarm.

Flavinuria: The Bright Yellow Urine

What it is: The most famous side effect is flavinuria, which is a bright, fluorescent yellow or orange color in the urine.
What causes it: This happens because the excess riboflavin, which has a natural yellow hue, is being passed out of the body through the kidneys.
Is it harmful?: Absolutely not. It is a completely harmless and expected outcome of taking high-dose supplements.

Gastrointestinal Upset

While uncommon, some individuals may experience mild gastrointestinal issues, especially when taking high-dose supplements, such as the 400 mg daily doses used for migraine prevention.

Reported symptoms: These can include nausea, diarrhea, and stomach cramps.
Frequency: Side effects like these are not universal and only affect some people.
Solution: Often, adjusting the dosage can alleviate these symptoms.

Potential Complications and Considerations

While serious toxicity is rare, there are a few important considerations for individuals on high-dose riboflavin.

Drug Interactions

Vitamin B2 can interact with certain medications, affecting their efficacy. It's crucial to discuss any supplement regimen with a healthcare provider.

Antidepressants: Tricyclic antidepressants like imipramine may be affected.
Antibiotics: Tetracycline is another medication that can be influenced by B2 supplements.
Cancer therapies: Certain cancer drugs, such as doxorubicin, may also interact with riboflavin.

High-Dose Photosensitivity: A Theoretical Risk

Some laboratory (in vitro) studies have shown that riboflavin can increase sensitivity to ultraviolet light by creating active oxygen species, but its relevance in a living body (in vivo) is highly questionable. Some sources mention light sensitivity, but it is not a widely confirmed clinical effect of excess oral intake.

Possible Nutrient Imbalances

Taking any single supplement in consistently high doses without medical supervision could potentially lead to imbalances with other vitamins or nutrients. A balanced diet or B-complex supplement is often recommended to maintain overall nutrient harmony.

High-Dose Riboflavin in Medical Contexts

High doses of riboflavin, often far exceeding the Recommended Daily Allowance (RDA), are sometimes medically supervised for therapeutic benefits. For example, 400 mg daily doses are often used as a prophylactic treatment for migraine headaches, with studies showing a modest reduction in frequency and severity. In some cases, it is also used to help manage elevated homocysteine levels, a risk factor for heart disease.

Dietary Intake vs. Supplemental Intake


Feature	Dietary Riboflavin	Supplemental Riboflavin
Sources	Milk, dairy, eggs, meat, green vegetables, fortified cereals	Tablets, capsules, B-complex formulas
Toxicity Risk	Extremely low to non-existent; almost impossible to get too much from food alone	Very low, but still higher risk of side effects than dietary sources
Side Effects	Rare or non-existent at typical intake levels	Mild side effects like bright yellow urine, nausea, or diarrhea are possible
Absorption	Highly efficient absorption of smaller amounts	Less efficient absorption; body absorbs a limited amount, excretes the rest
Purpose	To meet daily nutritional needs for metabolic function	Therapeutic use (e.g., migraines) or addressing deficiencies

Conclusion: The Final Word on Excess B2

In summary, experiencing side effects from excess vitamin B2 is unlikely for most people. Due to its water-soluble nature and the body's limited absorption capacity, any surplus is rapidly excreted. The most common and harmless sign is bright yellow urine. For those taking very high-dose supplements, minor issues like stomach upset might occur, but these are generally infrequent and mild. While true toxicity is not a concern, it is always wise to consult a healthcare provider before beginning a high-dose supplement regimen, particularly to discuss potential interactions with other medications.

For more detailed information on riboflavin, consult the Office of Dietary Supplements: Riboflavin Fact Sheet

Frequently Asked Questions

A toxic overdose of riboflavin is highly unlikely, even with high-dose supplements. The body’s ability to absorb it is limited, and any excess is naturally flushed out in the urine.

The most common and harmless sign of excess riboflavin is a vivid, fluorescent yellow or orange color in your urine. This is a normal and expected effect, not a cause for concern.

While severe side effects are not known, very high supplemental doses may cause mild symptoms in some people, such as nausea, diarrhea, or stomach cramps.

Riboflavin is water-soluble, meaning it dissolves in water and is not stored in the body's fat cells. Excess amounts are simply excreted in the urine. Additionally, the body has a limited capacity to absorb riboflavin at any one time.

No, it is virtually impossible to consume too much riboflavin from food sources alone. Adverse effects from dietary intake have not been reported.

There is no established Tolerable Upper Intake Level (UL) for riboflavin, as there is insufficient evidence to suggest adverse effects from high intakes in healthy individuals.

While not a toxicity risk, it is important to consult a doctor before taking high-dose B2 supplements, especially if you are on other medications, to check for potential drug interactions.

Consistently taking high doses of any single supplement without guidance could potentially lead to imbalances with other nutrients, including other B-vitamins.