Quick answer

NIH reports no known side effects from BCAA supplements at 20 g a day or less for up to 6 weeks. Larger amounts raise ammonia production, and a few groups should avoid them or ask first, including people taking levodopa and anyone born unable to break down these amino acids.^1,2,3,4

Key facts

NIH says up to 20 g a day, split into doses, appears to be safe.¹
Leucine cut levodopa absorption from the gut from about 40% to 21% in a study of 7 volunteers.⁵
The long-term effects of high doses of amino acid supplements still need study, a 2022 safety review says.²
The upper safe intake for leucine, about 500 mg per kg a day, was set from studies of men.¹

What studies report

NIH lists no known side effects and no safety concerns at 20 g a day or less for up to 6 weeks.¹

No tolerable upper intake level is set for BCAAs. NIH says up to 20 g a day, taken in divided doses, appears to be safe, and puts the upper safe intake of leucine alone at about 500 mg per kg of body weight a day, about 38 g for a 75 kg man, based on studies in men.^1,6

Large amounts of BCAAs raise the body's production of ammonia. A review of amino acid supplements says they may be harmful for people with liver disease and appear inappropriate for people with diabetes.²

Most of these studies lasted days or weeks, in healthy young adults, and many were small. That is enough to find common short-term problems, not rare or slow ones.^2,7

Who should not take BCAAs

People with maple syrup urine disease should not take BCAA supplements. In this inherited disorder, the enzyme complex that breaks down leucine, isoleucine and valine works poorly or not at all, so these amino acids and their byproducts build up to levels that are toxic to the brain and other organs. It usually shows up in infancy or childhood.⁴

Levodopa and amino acids

Ask your pharmacist or doctor first

Levodopa competes with certain amino acids for absorption from the gut, and its label says a high protein diet may reduce how much is absorbed. In studies, leucine cut levodopa absorption from the gut, and leucine or isoleucine taken by mouth reversed the effect of levodopa given by drip. The label does not name BCAA supplements, so ask your doctor or pharmacist before combining them.^3,5,8

Reported in studies

Levodopa is carried across the gut wall and into the brain by the same carrier as large neutral amino acids, which include leucine and isoleucine. In people whose response to levodopa rose and fell during the day, high-protein meals or oral leucine or isoleucine reversed its effect.^5,8

If you take levodopa, ask your doctor or pharmacist before using a BCAA product, and how it fits around your doses.

Who should ask a doctor first

Liver disease: large amounts of BCAAs raise ammonia, which a damaged liver clears poorly²
Diabetes: a safety review says BCAA supplements appear unsuitable²
ALS or another neurological or psychiatric condition²
Pregnancy or nursing: the safety of amino acid supplements has not been well studied²
Anyone under 18⁶

In a trial in 126 people with ALS, 24 g a day of BCAAs gave no benefit, and the trial was stopped early because more people died in the BCAA group (24) than on placebo (13).⁹

Is BCAA safe for teenagers?

NIH advises that in most cases only adults should use performance supplements, and cites the American Academy of Pediatrics' view that they do not help teen athletes beyond good nutrition and training. The BCAA studies described here were done mostly in young adults, so their safety findings cannot simply be carried over to a 13 or 16 year old.^6,7

Adding it up

NIH's figure of up to 20 g a day refers to BCAA supplements taken on top of a normal diet, which itself supplies 10 to 20 g. If you use more than one product with added BCAAs, add the amounts together.^1,6

Questions people ask

Are BCAAs safe for women?

The same few cautions apply, but the safety data come mostly from studies of men, including the work behind the leucine limit. If you are pregnant or nursing, ask your doctor first.^1,2

Is BCAA safe for people with diabetes?

Ask your doctor first. A review of amino acid supplements says BCAAs appear unsuitable for people with diabetes, and their suitability for people with medical conditions still needs study.²

Do BCAAs have side effects at normal amounts?

None are known at 20 g a day or less for up to 6 weeks, according to NIH. Larger amounts and longer use are less studied, and the cautions above apply to people with those conditions.^1,2

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Sources

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