Common Side Effects of Taking B2 (Riboflavin)

October 23, 2025 •

3 min read

According to research, high doses of vitamin B2 (riboflavin) are generally considered safe for most people, though they can cause mild side effects. A key side effect of taking B2 supplements is a bright yellow discoloration of the urine, which is harmless and occurs as the body excretes excess amounts. Other potential issues, such as digestive discomfort, are more likely with higher therapeutic doses.

Quick Summary

Taking riboflavin supplements often results in bright yellow urine, a harmless and expected side effect due to the vitamin's natural color and how the body excretes excess amounts. While larger therapeutic doses may cause some digestive upset, vitamin B2 is not considered toxic, and a tolerable upper intake level has not been established. Serious adverse reactions are rare, but allergic responses can occur.

Key Points

Bright Yellow Urine: Excess vitamin B2 is excreted, causing a harmless bright yellow discoloration of urine.
Digestive Issues Possible at High Doses: Some individuals taking very high doses for therapeutic reasons, such as for migraines, may experience mild nausea, diarrhea, or stomach cramps.
Limited Toxicity Risk: The body has a limited capacity to absorb riboflavin, and excess is excreted, making toxicity from oral intake very unlikely.
Rare Allergic Reactions: Although extremely uncommon, serious allergic reactions, including hives, swelling, and difficulty breathing, are possible.
No Tolerable Upper Intake Level (UL): Due to its low toxicity, an official UL has not been established for riboflavin, as adverse effects are not observed with high oral intakes.
Photosensitivity Concerns with Very High Intake: Extremely high doses may lead to increased sensitivity to sunlight, though this is a very rare and theoretical risk.

What is Vitamin B2 (Riboflavin)?

Vitamin B2, or riboflavin, is a water-soluble vitamin that plays a crucial role in various bodily functions, including energy production, cellular growth, and the metabolism of fats, carbohydrates, and proteins. Unlike fat-soluble vitamins, which can accumulate in the body, excess riboflavin is not stored and is instead excreted through the urine. This mechanism is the primary reason for one of its most notable, yet benign, side effects.

The Most Common Side Effects of Taking B2

Most people who take B2 supplements experience minimal, if any, negative side effects. The primary effect is a noticeable change in the color of urine, particularly with larger doses.

Bright Yellow Urine (Flavinuria)

This is the most common and widely recognized side effect of taking riboflavin. When you consume more B2 than your body needs, the excess is quickly excreted by the kidneys. Since riboflavin has a naturally bright yellow, fluorescent color, this gives urine a vivid, often startling, yellow-orange hue. This effect is entirely harmless and is simply a sign that your body is processing and removing the surplus vitamin. The color change will subside once supplementation stops.

Potential Gastrointestinal Discomfort

While less common, some individuals may experience minor digestive issues, especially when taking very high doses (e.g., 400 mg daily for migraine prophylaxis).

Nausea: A feeling of sickness in the stomach.
Diarrhea: This has been reported in clinical trials involving high doses of riboflavin.
Stomach cramps: Some people may experience mild abdominal cramping.

Allergic Reactions

Though very rare, an allergic reaction to riboflavin is possible, as with any supplement or medication. It is important to seek immediate medical attention if you experience symptoms such as:

Skin rash or hives
Itching or swelling (especially of the face, tongue, or throat)
Severe dizziness
Trouble breathing

Increased Sensitivity to Light (Photosensitivity)

Some reports have noted that very high doses of vitamin B2 could lead to increased sensitivity to light, a condition known as photosensitivity. This is not a common side effect but may occur with therapeutic doses. While rare, it's a theoretical risk that has been demonstrated in in vitro studies.

Side Effects by Dosage: Common vs. High-Dose

While the body safely excretes excess B2, the type and likelihood of side effects can vary depending on the dosage. Here is a comparison of typical versus high-dose supplementation.


Side Effect	Typical Daily Dose (<100 mg)	High Therapeutic Dose (400 mg)
Urine Discoloration	Mild to moderate bright yellow urine.	Pronounced bright yellow to fluorescent urine.
Gastrointestinal Issues	Highly unlikely and not commonly reported.	Possible but generally mild nausea or diarrhea.
Photosensitivity	Not a reported side effect.	Very rare, but a theoretical risk of increased light sensitivity.
Allergic Reaction	Extremely rare.	Extremely rare; immediate medical attention needed if symptoms occur.

The Science of B2 and Safety

One of the most reassuring aspects of riboflavin is its limited capacity for absorption. The gastrointestinal tract can only absorb a certain amount of the vitamin at a time. This natural limitation, combined with its rapid excretion by the kidneys, makes it nearly impossible to experience a toxic overdose from oral supplementation alone. For this reason, regulatory bodies have not established a tolerable upper intake level (UL) for riboflavin, as there is no evidence of toxicity from high oral intakes. This is in contrast to some other B vitamins, such as B3 (niacin) and B6 (pyridoxine), which can have adverse effects in large quantities.

Conclusion

For most people, the side effects of taking B2 are minimal and include the harmless change of urine color. While higher doses, often used therapeutically for conditions like migraines, may introduce mild gastrointestinal upset or, very rarely, allergic reactions, riboflavin has a strong safety profile. Its water-soluble nature ensures that any excess is efficiently removed from the body, preventing toxicity. As with any supplement, it is wise to consult a healthcare provider before starting B2, especially if taking high doses or other medications, to ensure it is appropriate for your individual health needs.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Please consult with a healthcare professional for personalized guidance.

Frequently Asked Questions

No, bright yellow or fluorescent urine from taking vitamin B2 supplements is completely harmless. It is simply the body's natural way of excreting excess riboflavin, a water-soluble vitamin.

An overdose of vitamin B2 is highly unlikely. The body's absorption of riboflavin is limited, and any unabsorbed or excess amounts are simply flushed out in the urine, preventing a toxic buildup.

While uncommon, some individuals may experience mild nausea or other digestive discomfort, especially when taking high therapeutic doses of riboflavin, such as 400 mg daily.

The bright yellow urine caused by excess riboflavin is temporary. Once you stop supplementation, the urine color should return to its normal shade relatively quickly, as the body expels the remaining excess vitamin.

Yes, some medications can interact with riboflavin. For instance, certain antidepressants and seizure medications may be affected. It is always important to discuss any supplements you are taking with your doctor or pharmacist.

For pregnant women, riboflavin is generally considered safe when taken in the amounts commonly found in food and recommended supplements. However, as with any supplement, consulting a healthcare provider is essential.

When taking the high doses of 400 mg daily often used for migraine prevention, side effects can include bright yellow urine, as well as mild digestive issues like nausea or diarrhea in some cases. Studies have noted few serious adverse effects from this use.