Quick answer
A systematic review of 71 human studies found nausea, stomachache and diarrhea were the main adverse events, but gut symptoms were not significantly more common with lysine than without it (risk ratio 1.02). Its provisional no-effect level was 6,000 mg a day, and no official upper limit exists. Ask your doctor if symptoms persist.1
Key facts
- A systematic review of 71 human studies found the adverse events were mainly nausea, stomachache and diarrhea.1
- In the pooled analysis, gut symptoms were not significantly more common with lysine than without it: risk ratio 1.02, 95% confidence interval 0.96 to 1.07.1
- The review proposed a provisional no-observed-adverse-effect level of 6,000 mg a day, based on gut symptoms, and no official upper limit exists.1
- The studies used 16.8 mg to 17,500 mg a day for 1 to 1,095 days, and only 12 of the 71 were of relatively high quality.1
- LactMed says caution is warranted with any single amino acid far above the level normally found in food.2
What did the systematic review find?
The review searched four databases for human studies of oral L-lysine added to an ordinary diet and selected 71 of them. It reports that the adverse events seen were mainly subjective symptoms of the digestive tract, such as nausea, stomachache and diarrhea.1
When the authors pooled the risk of developing digestive symptoms, the risk ratio was 1.02, with a 95% confidence interval of 0.96 to 1.07 and a p value of 0.49. In plain terms, the symptoms were not significantly more common with lysine than without it.1
The side effects reported in human studies were mainly nausea, stomach ache and diarrhea. Pooled across studies, these were no more common with lysine than without it.1
How much lysine did the studies use, and for how long?
The doses in the 71 studies ranged from 16.8 mg to 17,500 mg a day, and the dosing periods from 1 day to 1,095 days, which is about 3 years. Only 12 studies were of relatively high quality, with Jadad scores of 3 or more.1
Human studies of lysine supplements have used from about 17 mg to 17,500 mg a day, for anywhere from 1 day to 3 years.1
Because the range is so wide and most studies were small, the review cannot say how likely diarrhea is at any one dose. It does not rule out digestive upset in individual people.1
Is there an upper limit for lysine?
No official upper limit is set for lysine. A 2019 systematic review of 71 human studies proposed a provisional no-observed-adverse-effect level of 6,000 mg a day, based on gut symptoms.1
The sources checked for this page include no NIH fact sheet, NCCIH page or LiverTox entry on lysine supplements, so the review's number is a research proposal and not an official limit. LactMed, in its entry on lysine during breastfeeding, makes a general point that caution is warranted when using any single amino acid at a level significantly above that normally found in food.2
For comparison, the World Health Organization puts the adult lysine requirement from all food at about 30 mg per kg of body weight a day, which is roughly 2,100 mg for a 70 kg adult.3
Does lysine cause constipation or other problems?
The review's summary names nausea, stomachache and diarrhea. It does not name constipation, so the sources checked do not link lysine to it. That is not a finding either way.1
A 1996 case report described a 44-year-old woman who developed Fanconi syndrome and tubulointerstitial nephritis, serious kidney damage that progressed to chronic kidney failure, in association with taking lysine. It is a single case.4
Who should ask a doctor about lysine and digestive upset?
- Anyone with ongoing diarrhea, nausea or stomach pain while taking lysine.1
- Anyone with kidney disease or reduced kidney function, given a case report of serious kidney damage.4
- Anyone who is pregnant or breastfeeding, or under 18.2
- Anyone taking a prescription medicine, since the drug labels checked do not address lysine either way.
Ask your doctor, and bring the label of any lysine product so the amount per serving can be checked.
Questions people ask
Does lysine cause nausea?
The systematic review of 71 human studies lists nausea, stomachache and diarrhea as the main adverse events, but not significantly more common with lysine than without it. Ask your doctor if nausea persists.1
How much lysine causes diarrhea?
The review could not say. It proposed a provisional no-observed-adverse-effect level of 6,000 mg a day, based on gut symptoms, and no official upper limit exists. Ask your doctor.1
Can lysine upset your stomach?
The review found stomach symptoms were reported in studies but were not significantly more common with lysine than without it, with a risk ratio of 1.02. Individual people can still react, so ask your doctor.1
Does lysine cause constipation?
The review's summary names nausea, stomachache and diarrhea and does not name constipation. The sources checked do not link lysine to it, which is not a finding either way, so ask your doctor.1
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Sources
- Hayamizu K, Oshima I, Fukuda Z, et al. Safety assessment of L-lysine oral intake: a systematic review. Amino Acids. 2019;51(4):647-659. Opened 2026-10-07.
- Drugs and Lactation Database (LactMed). Lysine. National Institute of Child Health and Human Development. Record updated June 15, 2025. Opened 2026-10-11.
- Joint WHO/FAO/UNU Expert Consultation. Protein and amino acid requirements in human nutrition. World Health Organ Tech Rep Ser. 2007;(935):1-265. Opened 2026-10-07.
- Lo JC, Chertow GM, Rennke H, Seifter JL. Fanconi's syndrome and tubulointerstitial nephritis in association with L-lysine ingestion. Am J Kidney Dis. 1996;28(4):614-617. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.