Benefits
Cold sores: what the cited research actually found
This is the most common reason people buy lysine, and the evidence cited here does not support it. The single reference on this page is a 2015 Cochrane systematic review that pooled 32 randomized trials in 2,640 people with normal immune systems, and it reported failing to find any evidence of efficacy for lysine in preventing cold sores. An older controlled trial from 1987 did report success with L-lysine for recurrent herpes simplex, but that early positive result was not confirmed once later evidence was pooled. In the same review, the interventions that did help were prescription antiviral pills such as aciclovir and valaciclovir, and even there the reviewers described the clinical benefit as small and rated the evidence low to moderate quality. Cold sores are a viral infection, so anyone with frequent or severe outbreaks should talk to a clinician rather than rely on an amino acid.
A building block the body uses for collagen
Lysine is one of the amino acids the body chemically modifies to build the crosslinks that hold collagen fibers together, so getting enough of it from food is part of normal bone, skin and connective tissue maintenance. That is textbook nutrition rather than a benefit of supplementing. No study cited on this page tested lysine supplements for bone density, wound healing, skin or hair, or calcium absorption in people.
Carnitine synthesis
Lysine (along with methionine) provides the carbon backbone for endogenous carnitine synthesis. Getting enough lysine from food supports the body's own carnitine production, which cells use to move fatty acids for energy. This is a nutritional role only. No study cited on this page tested whether taking lysine supplements changes carnitine levels or energy in people.
Stress and anxiety: only tested combined with arginine
The study behind this claim did not test lysine on its own. It gave L-lysine together with L-arginine and reported lower anxiety scores and lower cortisol than placebo. Because every participant who received lysine also received arginine, the trial cannot show what lysine alone does for stress or mood, which matters because lysine is usually sold on its own. The serotonin receptor explanation often quoted for this effect is a proposed mechanism, not something the trial measured.
Mechanism of action
Arginine competition and antiviral activity
Lysine and arginine compete for the same intestinal transporters (CAT-1, CAT-2) and cellular uptake pathways. High lysine concentrations outcompete arginine for absorption, reducing intracellular arginine availability. The theory is that with less arginine available, the herpes virus, which needs arginine to copy itself, would be held back. It is important to know this remains a theory: when 32 randomized trials in 2,640 people were pooled in a 2015 Cochrane review, lysine showed no evidence of preventing cold sores.
Collagen crosslinking via hydroxylysine formation
Lysyl hydroxylase (LH1-3) hydroxylates specific lysine residues in procollagen chains to form hydroxylysine — a modification required for collagen fiber crosslinking via lysyl oxidase. Without adequate hydroxylysine formation, collagen fibers cannot form the stable intermolecular crosslinks needed for tensile strength in bone, tendons, and skin.
Proposed serotonin and cortisol pathway
Lysine has been proposed to interact with serotonin 5-HT4 receptors in the gut and brain, and this is the usual explanation offered for the calming and cortisol findings in the research. Two caveats: that research used lysine combined with arginine rather than lysine alone, and the receptor pathway itself was not measured in it. Treat this as an untested explanation.
Clinical trials
Two pieces of research read together. First, a controlled trial published in Dermatologica in 1987 that reported success with L-lysine for recurrent herpes simplex. Second, the 2015 Cochrane systematic review that pooled 32 randomized trials in 2,640 people with normal immune systems and assessed everything used to prevent cold sores, lysine included.
The 1987 trial studied people with recurrent herpes simplex; the 2015 review covered 2,640 people with normal immune systems across 32 randomized trials.
The 2015 Cochrane review reported that its authors failed to find any evidence of efficacy for lysine in preventing cold sores. Lysine was grouped with the interventions that showed no effect. What did work in that review was prescription antiviral medication such as aciclovir and valaciclovir, and even for those the reviewers called the clinical benefit small and rated the evidence low to moderate quality. So the honest chronology is an encouraging 1987 result that later pooled evidence did not confirm. Cold sores are a viral infection, and frequent or severe outbreaks are worth discussing with a clinician.
Randomized, double-blind, placebo-controlled trial of L-lysine (2.64 g/day) + L-arginine (2.64 g/day) vs placebo in 108 healthy adults under stress. Outcomes: STAI anxiety scores, salivary cortisol response. (Smriga et al. 2007, Biomed Res)
108 healthy adults.
The lysine plus arginine combination lowered state anxiety scores and the salivary cortisol stress response compared with placebo. Because everyone who received lysine also received arginine, this trial cannot tell you what lysine alone does, which is how lysine is normally sold. The mechanism behind the finding is speculative. Note: small effect size; not established anxiolytic; benzodiazepines and SSRIs remain first-line for clinical anxiety.