L-Lysine

What it is

Lysine is an essential amino acid, meaning your body can't make it and you get it from protein-rich foods. As a supplement, it's sold mainly as L-lysine hydrochloride.

How it works

The herpes virus that causes cold sores needs the amino acid arginine to replicate, and lysine competes with arginine in the body. The theory is that tipping the balance toward lysine makes it harder for the virus to reactivate and multiply. That works in lab studies, but it hasn't translated into consistent results in people.

What it's been studied for

Commonly taken to reduce how often cold sores come back, and sometimes to shorten outbreaks.

What the evidence shows

Mixed to weak: a Cochrane review of cold sore prevention found no clear evidence that lysine reduces recurrences. Some older, small trials suggested benefit at higher doses, and a later review concluded lower doses appear ineffective while higher doses may help some people, but the studies are few, dated and inconsistent.

Who should consider it

People with recurring cold sores who want a low-risk option to try alongside — not instead of — their provider's advice might reasonably consider it. People with kidney disease shouldn't use it without medical guidance. If outbreaks are frequent, severe or near the eyes, prescription antiviral medications have much stronger evidence and are worth discussing with your provider.

Amounts commonly studied

Studies typically used 1 to 3 grams per day, with the more encouraging results generally at the higher end.

How to take it

Studies typically used 1 to 3 grams per day, often split into doses and taken daily for months in the studies. It can be taken with or without food; splitting doses may reduce stomach upset. Talk to your provider before starting if you have kidney or liver disease, are pregnant, or are managing frequent outbreaks.

Forms & absorption

Available as capsules and tablets (usually L-lysine hydrochloride) and in topical creams, which are a separate product with even less research. Oral lysine is well absorbed and can be taken with or without food.

Food sources

Meat, poultry, fish, eggs, dairy (especially cheese and yogurt), beans, lentils, soy foods like tofu, and quinoa.

Safety & interactions

Usually well tolerated at studied doses; stomach upset and diarrhea are the most common side effects. There are rare reports of kidney problems with high-dose use, so people with kidney disease should avoid it unless their provider approves, and safety in pregnancy and breastfeeding isn't established. Frequent or severe outbreaks, sores near the eyes, or outbreaks with a weakened immune system need medical care — prescription antivirals are well proven.

Common questions

Does lysine work for cold sores?

The evidence is mixed. A Cochrane review found no clear benefit for preventing recurrences, though some small trials at higher doses were more encouraging. Prescription antivirals have much stronger support.

Should I take it every day or only during an outbreak?

Most studies tested daily use over months for prevention rather than short-term use during outbreaks. If you choose to try it, discuss a plan with your provider, especially if outbreaks are frequent.

Can I get enough lysine from food?

Most people who eat enough protein get plenty of lysine for general health. The amounts studied for cold sores are higher than a typical diet provides, which is why they're taken as supplements.