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Ingredient guide

Is l-lysine safe to take? Side effects and cautions

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

In studies, lysine has mostly caused mild gut complaints such as nausea and stomach ache, and pooled across 71 studies these were no more common than without it. A 2019 review proposed 6,000 mg a day as a provisional level without adverse effects. Anyone with reduced kidney function, or who is pregnant or breastfeeding, should ask a doctor first.1

Key facts

  • The 71 studies in the 2019 review used about 17 mg to 17,500 mg of lysine a day, for up to 3 years.1
  • No official upper limit has been set for lysine.1
  • Most people already get more lysine from food than they need, so a supplement adds to an intake that is already enough.2
  • The amphetamine and levothyroxine labels we checked on DailyMed do not mention lysine.3,4

Side effects reported in studies

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

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 review pooled the gut complaints across studies and found a risk ratio of 1.02, meaning no meaningful increase over no lysine. Its 6,000 mg figure is provisional because good studies were few: only 12 of the 71 scored as relatively high quality.1

Kidneys

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.5

One case cannot show that lysine caused the damage, but it is why people with kidney disease or reduced kidney function should check with a doctor before taking lysine at any dose.5

Lysine and cold sores

A Cochrane review of 32 trials of ways to prevent cold sores included lysine and found no evidence that it prevents them, so do not rely on it in place of medical care. Long-term antiviral medicines prescribed by a doctor did reduce recurrences in that review, though only modestly.5,6

Medicines

We found no NIH fact sheet, MedlinePlus medicine page, LiverTox entry or DailyMed label that documents an interaction between lysine supplements and a medicine. That is an absence of information, not proof that a combination is harmless, so ask your pharmacist about your own medicines.3,4

Who should ask first

  • People with kidney disease or reduced kidney function
  • Anyone pregnant or breastfeeding
  • Anyone under 18
  • Anyone taking prescription medicines

Questions people ask

Can I take l-lysine with amphetamine salts?

The amphetamine salts label we checked lists acidifying and alkalinizing agents that change amphetamine levels, but does not name lysine. A related stimulant, lisdexamfetamine, is built from dextroamphetamine and l-lysine, which the body separates in the blood. Neither label addresses a lysine supplement, so ask your pharmacist.3,7

Can I take l-lysine with thyroid medicine?

The levothyroxine label names iron and calcium supplements and antacids as products that can reduce its absorption, and does not mention lysine. That is not a finding either way, so ask your pharmacist how to fit any supplement around your thyroid dose.4

Is it safe to take l-lysine every day?

Studies have used daily lysine for up to 3 years, with side effects mainly mild gut complaints, but only 12 of the 71 studies in the 2019 review were of relatively high quality. Since most people already eat enough lysine, a daily supplement is rarely needed.1,2

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Sources

  1. 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.
  2. Berryman CE, Cheung SN, Collette EM, Pasiakos SM, Lieberman HR, Fulgoni VL 3rd. Amino Acid Intake and Conformance With the Dietary Reference Intakes in the United States: Analysis of the National Health and Nutrition Examination Survey, 2001-2018. J Nutr. 2023;153(3):749-759. Opened 2026-10-07.
  3. Adderall (dextroamphetamine saccharate, amphetamine aspartate, dextroamphetamine sulfate and amphetamine sulfate) tablets: prescribing information (Teva Pharmaceuticals USA). DailyMed, U.S. National Library of Medicine. Published June 18, 2024. Opened 2026-10-07.
  4. Synthroid (levothyroxine sodium) tablets: prescribing information (AbbVie). DailyMed, U.S. National Library of Medicine. Published February 29, 2024. Opened 2026-10-07.
  5. 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.
  6. Chi CC, Wang SH, Delamere FM, Wojnarowska F, Peters MC, Kanjirath PP. Interventions for prevention of herpes simplex labialis (cold sores on the lips). Cochrane Database Syst Rev. 2015;(8):CD010095. Opened 2026-10-07.
  7. Vyvanse (lisdexamfetamine dimesylate) capsules and chewable tablets: prescribing information (Takeda Pharmaceuticals America). DailyMed, U.S. National Library of Medicine. Published May 6, 2026. Opened 2026-10-07.

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