Quick answer
In the 48-week trials behind the prescription L-glutamine label, 21% of 187 people taking it reported constipation, against 18% of 111 on placebo. MedlinePlus names constipation as a side effect to report. NIH reports no side effects in adult exercise trials. Ask your doctor if you have liver or kidney disease.1,2,3
Key facts
- Constipation was the most common adverse reaction in the label's trials: 21% on L-glutamine against 18% on placebo.1
- Nausea (19% against 14%), headache (18% against 15%) and abdominal pain (17% against 16%) were also common, and so were cough, limb pain, back pain and chest pain.1
- The trials gave the powder at 10 to 30 g a day by body weight for 48 weeks to people aged 5 to 58 with sickle cell disease.1
- NIH reports no side effects in the exercise trials it summarizes, which used up to about 45 g a day for several weeks in adults.3,4
What the prescription label reports
The best numbers on glutamine and constipation come from the label of the prescription powder. Two placebo-controlled trials gave 187 people L-glutamine and 111 people placebo twice a day for 48 weeks. Constipation was the most common adverse reaction, but it was nearly as common on placebo.1
| Reaction | L-glutamine (187 people) | Placebo (111 people) |
|---|---|---|
| Constipation | 21% | 18% |
| Nausea | 19% | 14% |
| Headache | 18% | 15% |
| Abdominal pain | 17% | 16% |
| Cough | 16% | 14% |
| Pain in an arm or leg | 13% | 7% |
| Back pain | 12% | 5% |
| Chest pain | 12% | 8% |
Reading the table, constipation differs by only 3 percentage points, while pain in an arm or leg, back pain and nausea show larger gaps. The label itself cautions that rates from one trial cannot be compared directly with another's and may not reflect practice. About 3% stopped the medicine because of side effects, and none of the three deaths in the treatment group was judged related to it.1
What the trial group was like
These were people with sickle cell anemia or a related blood condition, aged 5 to 58, and most (63%) were already taking the medicine hydroxyurea. The doses were set by body weight at 10 to 30 g a day. Their results may not match those of a healthy adult taking glutamine for training.1
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.1,2
What NIH and MedlinePlus add
MedlinePlus's page on the prescription form lists constipation, nausea, headache, abdominal pain, cough and pain in the back or limbs as side effects to report if severe or persistent. NIH's exercise fact sheet describes trials of glutamine supplements in healthy adults and says no side effects were reported at up to about 45 g a day for 6 weeks.2,3
No side effects were reported in the exercise trials summarized by NIH, which used up to 45 g a day for several weeks.3,4
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.5
The sources checked do not give a rate for constipation in healthy people taking ordinary supplement amounts.3,5
Who should ask their doctor
Ask your doctor before taking glutamine if you have liver disease, a urea cycle disorder or kidney disease, because glutamine is broken down to ammonia and a safety review says high intakes are inappropriate with liver or kidney impairment. Ask your pharmacist if you already take the prescription form, since adding a supplement would raise the total.2,6
Questions people ask
Does glutamine cause constipation in people who take it as a supplement?
The sources checked report constipation only for the prescription powder, at 21% on L-glutamine and 18% on placebo. They give no rate for supplement users, so ask your doctor about your own case.1
Does L-glutamine cause nausea or headaches?
In the label's trials, nausea affected 19% on L-glutamine against 14% on placebo, and headache 18% against 15%. NIH reports no side effects in adult exercise trials.1,3
Does glutamine cause diarrhea, bloating or gas?
None of these is among the reactions the label lists in more than 10% of people. The sources checked do not address them further, which is not a finding either way.1
Does L-glutamine cause weight gain, acne or kidney stones?
The sources checked do not report these effects. NIH lists no known side effects for glutamine in the exercise trials, and that is not a finding either way for long-term use.3
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Sources
- L-glutamine oral powder: prescribing information (Camber Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Published June 11, 2026. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. L-glutamine (drug information). Last revised September 15, 2017. 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.
- 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.
- 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.
- Holeček M. Side effects of amino acid supplements. Physiol Res. 2022;71(1):29-45. Opened 2026-10-01.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.