L-lysine

Essential amino acid

ATC code: A16AA-L-LYSINE

Brand names – supplements: Doctor's Best L-Lysine, Source Naturals L-Lysine, NOW Foods L-Lysine, Solgar L-LysineSupplements are not tested in clinical trials and are not registered as medications.

Updated 24 Sep 2026 · Methodology

Indications

Cold sores (herpes labialis)

Adjunct

Many patients take L-lysine to reduce recurrent labial herpes episodes. A 2017 systematic review by Mailoo and Venkatakrishnan pooled about 12 studies, 7 of them RCTs: 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. The review advises caution in patients with cardiovascular disease and gallstones (theoretical risk via reduced arginine). With rare mild episodes, many dermatologists accept a trial of lysine alongside discussing the plan with the doctor.

Sources

ICD-10: B00.1 · ICD-11: 1F00.01

Stress and anxiety

Individual decision

L-lysine for anxiety and stress has been studied mainly in combination with L-arginine rather than on its own. Two small randomized, double-blind, placebo-controlled trials (108 and 29 participants) reported lower trait and state anxiety and reduced cortisol during psychosocial stress in healthy adults with elevated anxiety, using about 2.64 to 3 g of each amino acid per day. Both trials came from a single research group linked to an amino acid manufacturer and have not been independently replicated. has not substantiated any L-lysine health claim, including stress-related ones. For L-lysine as a standalone agent, benefit in anxiety is not established, and no harm is documented at the doses studied.

Sources

ICD-10: F41.9 · ICD-11: 6B0Z

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 nail beauty

Not recommended

On iHerb and Instagram L-lysine is promoted as a collagen building block for skin beauty, hair strength and nail growth. (2011, opinion 2063) authorised no health claim for L-lysine; there is no approved claim on skin, hair or nails. 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)

How to read tiers A–F

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit

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.

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

Pregnancy: can you take L-lysine?

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: can you take L-lysine?

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.

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Frequently asked

What is L-lysine used for?

L-lysine is evaluated for the following indications with varying evidence strength: Cold sores (herpes labialis) (evidence tier C), Skin, hair and nail beauty (evidence tier D), Stress and anxiety (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?

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.

Official sources

Direct links to regulator labels. Open in a new tab.

AEMPS / CIMA

Other drugs in this group

L-ornithine, L-theanine, L-tyrosine, Licorice root (Glycyrrhiza glabra, root and rhizome; including deglycyrrhizinated DGL), Lion's Mane Mushroom (Hericium erinaceus), Ademetionine, Chlorophyll (chlorophyllin copper-sodium complex), Beta-hydroxy beta-methylbutyrate (HMB)