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Is L-lysine safe while breastfeeding?

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

LactMed says lysine supplements are probably low risk during breastfeeding, but that no information exists on high doses such as those taken for herpes infections. A systematic review of 71 human studies found mostly gut complaints and proposed a provisional no-effect level of 6,000 mg a day. Ask your doctor first.1,2

Key facts

  • LactMed says lysine is a normal part of breastmilk and that lysine supplements are probably low risk during breastfeeding.1
  • LactMed says no information is available on high-dose lysine during breastfeeding, for example for herpes infections, and that caution is warranted with any single amino acid far above food levels.1
  • LactMed lists the recommended intake for nursing mothers as 52 mg per kg of body weight daily, or 3.6 g for a 70 kg person.1
  • A systematic review of 71 human studies, using about 17 mg to 17,500 mg a day, found side effects mainly nausea, stomach ache and diarrhea, not significantly more common than without lysine.2
  • A single case report linked lysine with serious kidney damage in a woman who was not nursing.3

What does LactMed say about lysine and breastfeeding?

LactMed calls lysine an essential amino acid that is normally present in breastmilk. It lists the recommended intake for nursing mothers as 52 mg per kg of body weight a day, or 3.6 g a day for a 70 kg person.1

It reports that no information exists on high-dose lysine supplements during breastfeeding, giving herpes infections as an example of why people take them. It adds that milk lysine does not follow the mother's dietary intake, so oral lysine might not markedly raise the amount in milk, and that caution is warranted with any single amino acid far above the level in food.1

LactMed's summary judgment is that lysine supplements are probably low risk during breastfeeding. It adds that an alternate drug may be preferred while nursing a newborn or preterm infant.1

What do studies of lysine safety show?

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.2

The review that found this covered 71 human studies, with amounts from about 17 mg to 17,500 mg a day for anywhere from 1 day to 3 years. Its authors proposed a provisional no-observed-adverse-effect level of 6,000 mg a day, based on gut symptoms. No official upper limit has been set for lysine.2

The sources checked do not say how many of those participants were breastfeeding. That limits what the review can say about nursing mothers, so ask your doctor.

Is there a kidney concern?

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.3

A single case report cannot show cause. It is a reason for anyone with reduced kidney function to ask a doctor before taking a lysine supplement, and the same goes for a nursing mother who has kidney disease.3

How does supplemental lysine compare with food?

The World Health Organization puts the adult lysine requirement at about 30 mg per kg of body weight a day, and a recommended intake roughly 24% higher. The nursing recommendation LactMed lists is higher still, at 52 mg per kg.1,4

In US survey data from 2001 to 2018, fewer than 1% of adults ate less than the average requirement for any essential amino acid, lysine included. The sources checked do not give a separate figure for nursing mothers.5

Who should ask a doctor about lysine while breastfeeding?

  • Anyone planning high-dose lysine, for example for repeated herpes infections, since LactMed has no information on such doses.1
  • Anyone with kidney disease or reduced kidney function.3
  • Anyone taking a prescription medicine, since no drug label checked addresses lysine supplements either way.
  • Anyone nursing a newborn or preterm infant, since LactMed says an alternate drug may be preferred then.1

Ask your doctor, and bring the label of any lysine product so the amount per serving can be checked.

Questions people ask

Can you take lysine while breastfeeding?

LactMed says lysine supplements are probably low risk during breastfeeding but that no information exists on high doses. Ask your doctor, especially for a newborn or preterm infant.1

Is lysine for cold sores safe while nursing?

LactMed says no information is available on high-dose lysine during breastfeeding, such as for herpes infections, and advises caution with any single amino acid far above food levels. Ask your doctor.1

Does lysine pass into breast milk?

LactMed says lysine is a normal part of breastmilk and that milk lysine does not follow the mother's dietary intake, so supplements might not markedly raise it.1

How much lysine is too much?

No official upper limit is set. A systematic review of 71 studies proposed a provisional no-observed-adverse-effect level of 6,000 mg a day, based on gut symptoms. Ask your doctor, particularly while nursing.2

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Sources

  1. Drugs and Lactation Database (LactMed). Lysine. National Institute of Child Health and Human Development. Record updated June 15, 2025. Opened 2026-10-11.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

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