Quick answer
In the exercise trials NIH reviewed, glutamine caused no reported side effects at up to 45 g a day for several weeks. In 48-week trials of a prescription glutamine powder at 10 to 30 g a day, headache, cough and stomach complaints each affected more than 1 in 10 people; most were nearly as common on placebo.1,2
Key facts
- Athletes took 28 g a day for 14 days with no reported harm in one study.3
- The body breaks glutamine down to ammonia, which the liver has to clear.4
- The prescription glutamine label says no drug interaction studies have been done.2
- There are no data on glutamine powder in pregnancy or while breastfeeding, the same label says.2
In exercise studies
No side effects were reported in the exercise trials summarized by NIH, which used up to 45 g a day for several weeks.1,5
Researchers note that very large single doses taken before exercise, about 0.9 g per kg of fat-free mass, are often poorly tolerated, while about 20 g was well tolerated in their trial.6
In the trial of runners in the heat, stomach and gut symptoms were low and no worse with glutamine, even at single doses of up to about 54 g. A review also reports that doses of up to 0.65 g per kg were tolerated by patients without abnormal ammonia levels.3,7
What the prescription powder's label lists
In 48-week trials of the prescription powder at 10 to 30 g a day, constipation, nausea, headache, abdominal pain, cough, and pain in the arms, legs, back or chest were each reported by more than 10 percent of people. Constipation, headache, abdominal pain and cough were nearly as common on placebo; limb, back and chest pain were more common on glutamine.2,8
| Side effect | Glutamine (187 people) | Placebo (111 people) |
|---|---|---|
| Constipation | 21% | 18% |
| Nausea | 19% | 14% |
| Headache | 18% | 15% |
| Abdominal pain | 17% | 16% |
| Cough | 16% | 14% |
| Pain in arms or legs | 13% | 7% |
| Back pain | 12% | 5% |
| Chest pain | 12% | 8% |
These trials enrolled people aged 5 to 58 with sickle cell disease, who took 10 to 30 g a day by weight for 48 weeks. That is up to ten times a typical supplement serving, in people with a serious blood disorder, so the figures are an upper bound rather than what a healthy adult should expect.2
Who should ask a doctor first
Glutamine is broken down to ammonia, and a safety review says high intakes are inappropriate for people with liver or kidney impairment.4
- Liver disease or a urea cycle disorder, because these conditions clear ammonia poorly4
- Kidney disease or reduced kidney function4
- Cancer, or cancer treatment: a safety review says glutamine's safety is unclear for people being treated for serious illness4
- Pregnancy or breastfeeding, where there are no data2,8
- Anyone under 18: NIH says that in most cases only adults should use performance supplements1
Medicines
No official source we checked, including NIH's fact sheets, DailyMed labels and MedlinePlus, documents an interaction between glutamine and a medicine, and the prescription label says no interaction studies have been done. That is a gap, not a clean bill of health. MedlinePlus tells people taking prescription L-glutamine to tell their doctor about every supplement they use, and the same advice applies the other way round.2,8
If you take the prescription powder, do not add a supplement on top without your prescriber's approval.
How much is too much
No tolerable upper intake level is set for glutamine. NIH reports no evidence of toxicity at 0.21 to 0.42 g per kg of body weight a day (15 to 30 g for a 154 lb person) and no safety concerns at about 45 g a day for 6 weeks. A review notes that single doses of about 20 to 30 g appear to be without ill effect in healthy adults.3,5
Questions people ask
Is L-glutamine safe for women?
The exercise trials included few women and reported no side effects. There are no data on the prescription powder in pregnancy or breastfeeding, so ask your doctor first if either applies.2,9
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Sources
- National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
- L-glutamine oral powder: prescribing information (Camber Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Published June 11, 2026. Opened 2026-10-07.
- Gleeson M. Dosing and efficacy of glutamine supplementation in human exercise and sport training. J Nutr. 2008;138(10):2045S-2049S. Opened 2026-10-01.
- Holeček M. Side effects of amino acid supplements. Physiol Res. 2022;71(1):29-45. Opened 2026-10-01.
- National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals. Updated April 1, 2024. Opened 2026-10-07.
- Ogden HB, et al. Acute L-glutamine supplementation does not improve gastrointestinal permeability, injury or microbial translocation in response to exhaustive high intensity exertional-heat stress. Eur J Sport Sci. 2022;22(12):1865-1876. Opened 2026-10-01.
- Pugh JN, et al. Glutamine supplementation reduces markers of intestinal permeability during running in the heat in a dose-dependent manner. Eur J Appl Physiol. 2017;117(12):2569-2577. Opened 2026-10-01.
- MedlinePlus, U.S. National Library of Medicine. L-glutamine (drug information). Last revised September 15, 2017. Opened 2026-10-01.
- Legault Z, Bagnall N, Kimmerly DS. The Influence of Oral L-Glutamine Supplementation on Muscle Strength Recovery and Soreness Following Unilateral Knee Extension Eccentric Exercise. Int J Sport Nutr Exerc Metab. 2015;25(5):417-426. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.