Quick answer

Five of the eight B vitamins have not been shown to cause harm, but three have adult upper limits: niacin (35 mg from supplements), vitamin B6 (100 mg) and folic acid (1,000 mcg). The common effects are usually harmless: bright yellow urine from riboflavin and skin flushing from niacin.^1,2,3

Key facts

Nicotinic acid typically causes flushing at 30 to 50 mg or more.³
Taking 1 to 6 g of vitamin B6 a day for 12 to 40 months can damage nerves.⁴
Thiamin, riboflavin, pantothenic acid, biotin and vitamin B12 have no set upper limit, for lack of data on harm.^1,5
Biotin, often part of a B complex, can skew some lab results, including thyroid tests.^6,7

Common effects that are usually harmless

Riboflavin is yellow, and the body passes what it does not absorb or use in urine, so urine can turn bright yellow.²

Nicotinic acid, one form of niacin, typically causes flushing at 30 to 50 mg or more: the skin of the face, arms and chest turns red and can burn, tingle and itch. It can start within 30 minutes and usually passes. It is unpleasant rather than harmful, but it can come with headache, rash, dizziness or a drop in blood pressure.^3,8

Taking nicotinic acid with food, raising the amount slowly, or giving the body time to adjust can reduce flushing.³

Upper limits, and what happens above them

For adults, the upper limits are 35 mg a day of niacin and 1,000 mcg a day of folic acid from supplements and fortified foods, and 100 mg a day of vitamin B6 from all sources. No upper limit is set for thiamin, riboflavin, pantothenic acid, biotin or vitamin B12, for lack of data, though caution may still be warranted above recommended amounts. In 2023 Europe's food safety authority set a lower limit for vitamin B6 of 12 mg a day.^1,3,4,9

Upper limits are not targets. They mark the most that is unlikely to harm almost anyone, and they count every source: supplements and fortified foods for niacin and folic acid, and food as well for vitamin B6.¹

Taking 1 to 6 g of vitamin B6 a day for 12 to 40 months can cause severe nerve damage with loss of control of body movements, which usually stops when the supplement is stopped. Too much B6 can also cause painful skin patches, sensitivity to sunlight, nausea and heartburn.^4,10

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.^3,8

Large amounts of folic acid can correct the anemia caused by a vitamin B12 deficiency while the nerve damage continues, which can delay the diagnosis, and may make the deficiency worse. This is the basis of the upper limit for folic acid.^9,11

Taking a B complex every day

Daily intake is how B vitamins are meant to arrive, since the body keeps only small stores of most of them. Problems come from stacking: a multivitamin, a B complex and a separate B6, niacin or folic acid product can together pass a limit, so add up the panels.^1,12

Lab tests

B complex products can contain biotin, which above the recommended intake can make some lab tests, including thyroid and heart tests, read falsely high or falsely low.^6,13

Tell your doctor and the lab what you take before blood work. Thyroid tests are a documented example: the levothyroxine label asks patients to stop biotin-containing supplements before thyroid testing.^7,13

Medicines

Ask your pharmacist or doctor first

The methotrexate label says products containing folic acid may make methotrexate less effective when it is used for cancer, and that patients should take them only when their healthcare provider directs. For other uses, the label tells prescribers to give folic acid or folinic acid to lower the chance of side effects.^9,14

On the medicine's label

Ask your pharmacist or doctor first

Anticonvulsants such as phenytoin, carbamazepine and valproate can lower folate and vitamin B6 levels, and long-term use has been linked to lower biotin. Folate supplements might lower blood levels of these medicines, and 200 mg a day of vitamin B6 lowered blood levels of phenytoin and phenobarbital.^4,6,9

Reported in studies

Your doctor may want to check on you

Metformin may lower vitamin B12 levels, possibly by reducing its absorption. The metformin label recommends measuring vitamin B12 every 2 to 3 years in people who take it.^15,16

On the medicine's label

Your doctor may want to check on you

Daily use of acid-reducing medicines such as omeprazole over a long time, for example more than 3 years, can reduce how much vitamin B12 the body absorbs from food. The omeprazole label notes rare reports of B12 deficiency.^16,17

On the medicine's label

Ask your pharmacist or doctor first

B complex supplements can contain biotin. The levothyroxine label says biotin interferes with thyroid hormone tests built on biotin and streptavidin, which may give wrong results, and tells patients to stop biotin and any biotin-containing supplement for a time before thyroid function testing.^6,7

On the medicine's label

NIH also notes that cycloserine and theophylline can lower vitamin B6, isoniazid interferes with niacin, sulfasalazine reduces folate absorption, and furosemide and fluorouracil can lower thiamin. Ask your pharmacist if you take any of them.^3,4,9,12

Questions people ask

Is bright yellow urine from B complex harmful?

No. The color comes from riboflavin, and NIH describes discolored urine as a minimal side effect.²

Are B complex side effects different for women and men?

The upper limits are the same for adult men and women, and they do not change in pregnancy or breastfeeding. What changes in pregnancy is the recommended amount, so ask your doctor before adding B vitamins on top of a prenatal vitamin.¹

Can B complex cause numbness or tingling?

Very high vitamin B6, 1 to 6 g a day for 12 to 40 months, can damage nerves. Check the B6 on your label against the 100 mg US limit, and see a doctor about numbness or tingling, which can also be a sign of low vitamin B12.^1,4,18

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Sources

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