L-Lysine

Essential amino acid

ATC code: A16AA-L-LYSINE (L-Lysine)

Brand names – supplements

Doctor's Best L-Lysine, Source Naturals L-Lysine, NOW Foods L-Lysine, Solgar L-Lysine

Supplements are not tested in clinical trials and are not registered as medications.

Mechanism of action

Essential amino acid. It competes with arginine for the same transmembrane transporter. Herpes simplex virus type 1 requires arginine for replication, so lysine may theoretically reduce arginine availability in infected cells and slow viral replication. This is the basis for prophylaxis of recurrent labial herpes. Lysine also participates in collagen and carnitine synthesis, but no RCT shows that supplementation improves skin or hair status.

Indications

Herpes labialis

Adjunct

Many patients take L-lysine to reduce recurrent labial herpes episodes. A 2017 systematic review by Mailoo and Venkatakrishnan pooled seven small RCTs and observational studies: doses of 1–3 g/day reduced episode frequency or severity, but the meta-analysis was weak because of heterogeneous protocols and short follow-up. Herpes guidelines do not include lysine as standard. The standard for severe frequent recurrence remains antiviral aciclovir or valaciclovir. With rare mild episodes, many dermatologists accept a trial of lysine alongside discussing the plan with the doctor.

Sources

Integrative Medicine: Lysine for Herpes Simplex Prophylaxis: A Review of the Evidence (2017)

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

Skin, hair and nails (supplement marketing)

Not recommended

On iHerb and Instagram L-lysine is promoted as a collagen building block for skin beauty, hair strength and nail growth. reviewed the proposed health claims for L-lysine in 2011 and did not establish a cause-and-effect relationship between supplementation and maintenance of normal skin or connective tissue. There are no controlled studies for aesthetic indications. In a patient without nutritional deficiency (balanced diet with animal or plant protein) the supplement will not produce a visible effect on skin or hair. If hair loss or brittle nails bother the patient, discuss iron, zinc, vitamin D and thyroid function with the doctor.

Sources

EFSA: Scientific Opinion on the substantiation of health claims related to L-lysine (2011)

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Official sources

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AEMPS / CIMA

Spanish Medicines Information Centre (CIMA)

Safety

Contraindications

Hypersensitivity to lysine
Hyperlysinaemia and other inherited disorders of lysine metabolism
Severe renal failure

Serious adverse effects

Tubulointerstitial nephritis with prolonged high doses (isolated reports)
Hypercalcaemia when combined with high-dose calcium

Common adverse effects

Abdominal pain and diarrhoea at doses above 3 g/day
Nausea

PregnancyFDA C

FDA Category C. Safe in food-derived amounts. Safety of pharmacological doses in pregnancy has not been studied. If prophylaxis of herpes recurrence in pregnancy is considered, the decision is taken with the obstetrician and dermatologist.

Breastfeeding

Compatible. Lysine is naturally present in human milk and dietary protein. Supplemental doses do not cause clinically significant infant exposure.

Calculate RID

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Frequently asked

What is L-Lysine used for?

L-Lysine is evaluated for the following indications with varying evidence strength: Herpes labialis (evidence tier C), Skin, hair and nails (supplement marketing) (evidence tier D). See the full indication matrix with dosing and citations above on this page.

What are the side effects of L-Lysine?

Common side effects of L-Lysine (≥ 1 in 100): Abdominal pain and diarrhoea at doses above 3 g/day, Nausea. See the Safety section for uncommon and serious reactions.

Is L-Lysine safe during pregnancy?

FDA category C. FDA Category C. Safe in food-derived amounts. Safety of pharmacological doses in pregnancy has not been studied. If prophylaxis of herpes recurrence in pregnancy is considered, the decision is taken with the obstetrician and dermatologist.

Is L-Lysine compatible with breastfeeding?

Compatible. Lysine is naturally present in human milk and dietary protein. Supplemental doses do not cause clinically significant infant exposure.

Who should not take L-Lysine?

L-Lysine is contraindicated in: Hypersensitivity to lysine; Hyperlysinaemia and other inherited disorders of lysine metabolism; Severe renal failure. Full list in the Safety section.

Mechanism of action

Indications

Herpes labialis

Adjunct

Sources

Integrative Medicine: Lysine for Herpes Simplex Prophylaxis: A Review of the Evidence (2017)

Safety

Contraindications

Hypersensitivity to lysine
Hyperlysinaemia and other inherited disorders of lysine metabolism
Severe renal failure

Serious adverse effects

Tubulointerstitial nephritis with prolonged high doses (isolated reports)
Hypercalcaemia when combined with high-dose calcium

Common adverse effects

Abdominal pain and diarrhoea at doses above 3 g/day
Nausea

PregnancyFDA C

Breastfeeding

Compatible. Lysine is naturally present in human milk and dietary protein. Supplemental doses do not cause clinically significant infant exposure.

Calculate RID

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Frequently asked

What is L-Lysine used for?

What are the side effects of L-Lysine?

Common side effects of L-Lysine (≥ 1 in 100): Abdominal pain and diarrhoea at doses above 3 g/day, Nausea. See the Safety section for uncommon and serious reactions.

Is L-Lysine safe during pregnancy?

Is L-Lysine compatible with breastfeeding?

Compatible. Lysine is naturally present in human milk and dietary protein. Supplemental doses do not cause clinically significant infant exposure.

Who should not take L-Lysine?

L-Lysine is contraindicated in: Hypersensitivity to lysine; Hyperlysinaemia and other inherited disorders of lysine metabolism; Severe renal failure. Full list in the Safety section.