L-Lysine for Cold Sores and Herpes: What the Studies Actually Show
Written by Celso Bufano. Updated October 16, 2026. Every health fact links to its source.
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Short answer: the evidence for L-lysine is weak and mixed. Doses under 1 gram a day did not seem to work. A few small studies found fewer outbreaks at about 1.25 grams or 3 grams a day, but other studies at similar doses found nothing. Lysine did not help heal a cold sore that had already appeared. If you want to try it, talk to your doctor first — and know that prescription antivirals have much stronger evidence (MedlinePlus).
What is L-lysine?
Lysine is an amino acid — a building block of protein — that you get from food. It is also sold as a supplement, often marketed for cold sores. The idea behind it is that lysine and another amino acid, arginine, compete in the body, and that the herpes virus needs arginine.
What the studies found
The best summary we found is a 2017 review by Mailoo and Rampes, which looked at 12 studies, only 8 of them with a placebo (source).
| Daily dose | What the studies found |
|---|---|
| Under 1 g | Did not seem to work, especially without a low-arginine diet |
| About 1 g to 1.2 g | Mixed. Several double-blind trials found no significant effect |
| 1.25 g | One small trial (41 people) found fewer outbreaks; half that dose did not |
| 3 g | One small trial (59 people) found fewer outbreaks and milder symptoms |
| For a sore you already have | Two controlled trials found no benefit |
In their conclusion, the authors write that lysine “appears to be ineffective for prophylaxis or treatment of herpes simplex lesions with doses of less than 1 g/day,” and that longer, better studies at higher doses are needed.
Why the evidence is weak
The review points out that most studies were small, short, had many dropouts, and some were not controlled or relied on what patients remembered. Several also combined lysine with a low-arginine diet, so it is hard to know which one made the difference.
Is lysine safe?
The review says oral lysine toxicity has not been reported in humans, and that doses up to 3 grams a day are thought to be harmless. It also advises caution for people with heart disease, gallstones, asthma, erectile problems or immune problems, because lowering arginine may affect them.
Ask your doctor before taking lysine if you have any health condition, take medicine, or are pregnant or breastfeeding.
Should you try it?
- If you get frequent cold sores, see a doctor first. Daily antiviral tablets are the proven option (MedlinePlus).
- If you still want to try lysine, the studies suggest that low doses are unlikely to do anything.
- Don’t expect it to heal a sore that is already there. For that, read how to get rid of cold sores fast.
What about supplements made for herpes?
Some products sold for outbreaks don’t publish their full formula, so you can’t tell whether they contain lysine or how much. We compare two of them in best cold sore supplements.