Does Lysine Actually Prevent Cold Sores? What the Trials Show

By Erin Rose · Updated July 20, 2026 · Methodology

Educational summary — not medical advice. Lysine is not a proven cold-sore preventive, not a treatment for an active sore, and not a substitute for antiviral medication. This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.

Six trials, side by side

The only honest way to read this evidence is all six trials together, not the positive ones cited alone (which is how most marketing copy handles it). Positive result first, primary endpoint stated before any secondary finding, null result stated as plainly as the positive ones.

Simon 1985 has no abstract available on PubMed, so its exact n, dose, and effect size cannot be extracted or cited — it's included here by title, design, and null-result direction only, never with invented numbers.

Why the trials disagree

No single factor cleanly explains the split, and that uncertainty is itself part of the honest picture. A few real differences stand out. Dose isn't a clean predictor: DiGiovanna 1984's null result used 1,200 mg/day, inside the range that worked in McCune's 1,248 mg/day arm — undercutting any simple "just take enough" explanation. Absorption may matter more than the label dose: Thein & Hurt 1984 found the effect tracked a serum lysine threshold (~165 nmol/ml), not just whether someone took the pill, which could mean some "null" results reflect participants who never reached an effective blood level even at an on-paper-adequate dose. Every trial is small (n=21–65) and old (1978–1987), with different "recurrence" definitions and follow-up lengths, and none has been replicated in a large modern RCT — so some of the disagreement is plausibly statistical noise across underpowered studies rather than a real biological inconsistency.

The mechanism: real biology, not proof it works in people

The proposed reason lysine might help at all is the lysine/arginine antagonism: lysine structurally resembles arginine, an amino acid HSV needs to replicate, and in tissue-culture experiments lysine antagonized arginine's virus-growth-promoting effect (Griffith 1981). That's a real, published in-vitro finding — but it's cell-culture biology, not a human clinical outcome, and this page treats it strictly as rationale for why the trials above were worth running, never as evidence on its own that lysine works. One more citation-diversity note worth stating plainly: Robert S. Griffith authored both an earlier uncontrolled 1978 open study (Griffith 1978, no placebo arm, n=45) and the controlled 1987 RCT that is the strongest positive result in this evidence base — the same investigator behind the most favorable published results, which is a fair caveat about citation diversity, not an accusation of misconduct.

Cochrane's closing verdict — the most authoritative word here

The 2015 Cochrane systematic review (Chi et al.) assessed 32 RCTs covering 19 different interventions for preventing herpes labialis — the largest, most independent synthesis of this literature. Its finding: "long-term use of oral antiviral agents reduced the recurrence of HSL" with a small clinical benefit and no increased adverse events, while "topical antiviral agents and other interventions either showed no efficacy or could not confirm their efficacy in preventing HSL." Lysine falls in that second, weaker bucket — not the antiviral bucket. That's the single most authoritative sentence available about where lysine's cold-sore-prevention evidence actually stands in 2026: unconfirmed, not disproven, and clearly a notch below the treatments (oral antivirals) that Cochrane found do work.

Verified L-lysine products

If you've decided the low-risk experiment is worth trying at the trial-matched dose, here's how the L-lysine products we track and verify compare.

Frequently asked questions

Related

• L-lysine: what it is & the honest evidence
• Dosage guide — why 500 mg once a day probably isn't the trial dose
• L-lysine for cold sores — how people actually use it
• Best L-lysine — ranked by cost at the trial-relevant dose

Sources