Quick answer

NIH's riboflavin fact sheet reports that side effects of high-dose riboflavin, such as discolored urine, are minimal. Riboflavin, vitamin B2, is yellow, and the body passes in urine what it cannot absorb or use. It absorbs little from single doses beyond 27 mg. Riboflavin is one of the eight B vitamins in a B complex.^1,2

Key facts

Riboflavin is yellow, and what the body does not absorb or use is passed in urine, so urine can turn bright yellow.¹
The body absorbs little riboflavin from single doses beyond 27 mg and stores only small amounts.¹
In discussing a 400 mg a day dose, the NIH sheet reports that side effects such as discolored urine are minimal, and no upper limit is set for riboflavin.¹
MedlinePlus says there is no known poisoning from riboflavin.²
Other B complex effects NIH documents are niacin flushing, and nausea and heartburn at very large vitamin B6 intakes.^3,4

Why does B complex turn urine bright yellow?

The color comes from riboflavin, vitamin B2. NIH says riboflavin is yellow, that most of it in food is in two coenzyme forms, and that when extra is consumed it is either not absorbed or absorbed in small amounts and excreted in urine. The body takes up little from single doses beyond 27 mg.¹

MedlinePlus puts it the same way: riboflavin is water soluble, leftover amounts leave the body in the urine, and there is no known poisoning from riboflavin.²

In discussing a 400 mg a day dose, the NIH sheet reports that side effects such as discolored urine are minimal. It also explains that because adverse effects of high riboflavin intakes have not been reported, no upper limit was set, while noting that limited data do not mean high intakes have no adverse effects, and urging caution with excessive amounts.¹

What if urine looks a different color?

The sources describe only the bright yellow from riboflavin. They do not cover dark, brown, red or cloudy urine, and they do not say that a vitamin explains such a change. Changes of that kind are a reason to ask a doctor rather than to assume the supplement is the cause.¹

Does B complex cause nausea, diarrhea, constipation or headaches?

The sources document specific effects for specific B vitamins at specific amounts, rather than for B complex in general.^3,4

Niacin: nicotinic acid typically causes flushing of the face, arms and chest at 30 to 50 mg or more, and it can come with headache, rash, dizziness or a drop in blood pressure.^3,5
Vitamin B6: 1 to 6 g a day for 12 to 40 months can cause severe nerve damage, and too much can also cause nausea and heartburn.^4,6
High-dose niacin as a medicine: 1,000 mg a day or more of nicotinic acid can cause nausea and heartburn, and nicotinamide at 500 mg a day or more can cause diarrhea and easy bruising.^3,5
Constipation, gas, acne and weight gain: the sources checked do not address them, which is not a finding either way.^3,4

Nicotinic acid at 1,000 mg a day or more, taken as a medicine, can cause low blood pressure, extreme tiredness, high blood sugar, nausea, heartburn and, over months or years, liver damage. Nicotinamide at 500 mg a day or more can cause diarrhea and easy bruising.^3,5

Who should ask a doctor about B complex side effects

Anyone with flushing, nausea, numbness or tingling in the hands or feet, or a skin rash after starting a B complex product, who should ask their doctor or pharmacist.^3,4
Anyone taking more than one product that contains vitamin B6 or niacin, such as a multivitamin plus a B complex, who should ask a pharmacist to add them up.^3,4
Anyone with blood tests ahead who takes a product containing biotin, which can change some results.^7,8

Questions people ask

Is yellow urine from B complex normal?

NIH's riboflavin sheet reports that side effects of high-dose riboflavin, such as discolored urine, are minimal. Ask your doctor if the color is dark, brown or red, or comes with pain or other symptoms.¹

Does B complex cause nausea?

NIH reports nausea and heartburn with very large vitamin B6 intakes and with high-dose nicotinic acid, a form of niacin. These are large amounts, 1 to 6 g a day of vitamin B6 over months or 1,000 mg or more of nicotinic acid, so check the label against them.^3,4

Does B complex cause diarrhea?

NIH reports diarrhea with nicotinamide at 500 mg a day or more, a niacin form used as a medicine. Other B complex effects on digestion are not described in the sources checked.³

Does B complex cause constipation, weight gain, acne or gas?

The sources checked do not address these effects, which is not a finding either way. Ask your doctor or pharmacist if a symptom follows a new supplement.^3,4

Does B complex cause headaches?

NIH says nicotinic acid flushing can come with headache, rash, dizziness or a drop in blood pressure. Other B vitamins are not described as causing headaches in the sources checked.³

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