L-Lysine for Cold Sores: How It's Used (and What Serum Levels Mean)
Educational summary — not medical advice. Lysine is not a treatment for an active cold sore and not a substitute for antiviral medication; the underlying prevention evidence is genuinely mixed, not proven. This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.
The honest answer
The way lysine is actually studied — and the only way it showed any benefit — is daily prophylactic use at 1,000–3,000 mg/day, sustained for months, not taken only when a sore starts. It's aimed at reducing how often and how badly future sores recur, not clearing one you already have. And Thein & Hurt 1984 found the effect tracked a specific blood-level threshold, not just whether you took a pill — which helps explain why it doesn't work the same for everyone, even at the same labeled dose.
The protocol the positive trials actually used
All three positive trials in this evidence base — Griffith 1987, McCune 1984, and Thein & Hurt 1984 — had participants take lysine every day, not only when they felt a sore coming on. Griffith's trial, the strongest positive result, ran for 6 months at 1,000 mg three times daily; Thein & Hurt's ran for a full 12 months at 1,000 mg/day. That's a meaningfully different protocol than the popular "take it at the first tingle" advice you may have seen — and it matters, because no trial in this evidence set tested whether a single dose taken after symptoms start does anything for that specific episode. If you're going to try lysine on the strength of this evidence, the honest version of "how people actually use it" is a daily habit sustained for months, evaluated against your own recurrence history over that time — not a rescue remedy.
What the serum-threshold finding means in practice
Thein & Hurt 1984's most useful contribution isn't just that lysine "worked" in their trial — it's why. They found that when a participant's serum lysine exceeded roughly 165 nmol/ml, recurrence dropped significantly; below that threshold, the label dose alone wasn't doing the same job. That's a real, dose-and-absorption-dependent relationship, not a simple on/off switch from taking a pill. Practically, it means: (1) consistency matters more than any single dose — missing days and compensating with a bigger dose later is not the same as sustaining an elevated serum level; (2) individual absorption varies, so the same labeled dose won't put everyone at the same blood level, which is a plausible, evidence-grounded reason lysine "works for some people and not others" even within the positive-trial dose range; and (3) this finding has not been independently replicated, so treat it as the best available explanation for the inconsistency, not a confirmed biomarker you should test for.
The one-line takeaway If you try lysine for cold sores, the trial-matched way to do it is 1,000–3,000 mg/day, every day, for months — not an occasional pill when a sore starts. And go in knowing three equally rigorous trials at similar doses found nothing, so this is a low-risk experiment worth trying at the right dose, not a guaranteed fix.
Prevention, not treatment — and where antivirals fit in
It's worth being precise about what "for cold sores" means here. Every positive trial measured prophylaxis: fewer, milder, or faster-healing recurrences over months of sustained daily use. None measured whether lysine shortens or clears a sore that's already present — that's a different clinical question, and lysine has no trial evidence supporting it. If you have an active outbreak right now, the medications with actual evidence for treating it are antivirals — acyclovir, valacyclovir, and topical docosanol are the standard options, and the Cochrane review referenced throughout this cluster found oral antivirals genuinely reduce recurrence, unlike lysine. This page isn't the place to evaluate those drugs in depth; the point is simply that lysine and antivirals are answering different questions (fewer future sores vs. treating one now), and reaching for lysine mid-outbreak, instead of an antiviral, is a mismatch between the tool and the problem.
Who this is (and isn't) a reasonable experiment for
- Reasonable to try: people with recurrent cold sores who want a low-cost, low-risk daily prophylactic to test over several months, understanding the evidence is genuinely mixed.
- Not the right tool: anyone hoping to shorten or clear a sore they have right now — that's what antivirals are for.
- Not a guaranteed result: three equally rigorous trials at similar doses found no benefit — go in with realistic expectations, not a promise.
- Kidney disease? Get clinician input before high-dose amino acid supplementation — see the dosage guide's safety section.
Verified L-lysine products
If you're trying the daily prophylactic protocol described above, here's how the L-lysine products we track and verify compare.
| Product | Dose/Serving | Servings | Price | Cost/Day | Certification | Buy |
|---|---|---|---|---|---|---|
| Nutricost L-Lysine Powder (500 g) Best Value | 1000mg L-lysine | 500 | $15.95 | $0.10 | Manufactured in a GMP facility | Buy on Amazon |
| BulkSupplements.com L-Lysine Powder (500 g) | 1000mg L-lysine | 500 | $19.97 | $0.12 | Manufactured in a GMP facility | Buy on Amazon |
| Nutricost L-Lysine 1,000 mg (240 Tablets) | 1000mg L-lysine | 240 | $19.95 | $0.25 | Manufactured in a GMP facility | Buy on Amazon |
| Solgar L-Lysine 1,000 mg (250 Tablets) | 1000mg L-lysine | 250 | $23.01 | $0.28 | Manufactured in a GMP facility | Buy on Amazon |
| Nature's Bounty L-Lysine 1,000 mg (60 Tablets) | 1000mg L-lysine | 60 | $7.42 | $0.37 | Manufactured in a GMP facility | Buy on Amazon |
Frequently asked questions
Should I take lysine every day or only when a sore starts?
Every day, per the actual protocol in all three positive trials (months of sustained daily use). No trial tested as-needed dosing at the first tingle.
What does the serum-level finding mean for how I take it?
Thein & Hurt 1984 found benefit tracked a ~165 nmol/ml serum threshold, not just the label dose — meaning consistency and individual absorption matter more than any single dose.
Will lysine clear up my current cold sore?
No — every positive trial tested prevention of future recurrences, not treatment of an existing sore. Antivirals (acyclovir, valacyclovir, docosanol) are the treatments for that.
How long before I'd know if it's working?
The positive trials ran 6-12 months. A single sore-free month proves little, since cold sores recur intermittently even without intervention.
Related
- L-lysine: what it is & the honest evidence
- Dosage guide — the 1,000-3,000 mg/day range and a dose checker
- Best L-lysine — ranked by cost at the trial-relevant dose
- Does lysine actually prevent cold sores? — the full trial reckoning
Sources
- Griffith RS, Walsh DE, Myrmel KH, et al.. "Success of L-lysine therapy in frequently recurrent herpes simplex infection. Treatment and prophylaxis." Dermatologica. 1987. PMID: 3115841
- McCune MA, Perry HO, Muller SA, O'Fallon WM. "Treatment of recurrent herpes simplex infections with L-lysine monohydrochloride." Cutis. 1984. PMID: 6435961
- Thein DJ, Hurt WC. "Lysine as a prophylactic agent in the treatment of recurrent herpes simplex labialis." Oral Surg Oral Med Oral Pathol. 1984. PMID: 6438572
- Chi CC, Wang SH, Delamere FM, Wojnarowska F, Peters MC, Kanjirath PP. "Interventions for prevention of herpes simplex labialis (cold sores on the lips)." Cochrane Database Syst Rev. 2015. PMID: 26252373 (oral-antiviral efficacy contrast).