L-lysine: what it is and what the evidence says

L-lysine (usually just called lysine) is an essential amino acid — one of the building blocks of protein that your body cannot make for itself, so you have to get it from food. Most people who eat enough protein get plenty. As a supplement, lysine is most often promoted for cold sores, but the evidence there is mixed and not conclusive. This guide explains what lysine does, where you get it, and what the science actually shows.

This is general information, not personalised medical advice. If you have a health condition or take medication, speak to a doctor or pharmacist before starting a supplement.

What is lysine?

Lysine is one of the nine essential amino acids. "Essential" means your body can't produce it, so it must come from your diet. It contributes to the normal roles amino acids play — building protein for muscle and connective tissue, helping the body produce carnitine (involved in turning fat into energy), and supporting calcium use and immune function.

Because it's a normal part of dietary protein, lysine is not an exotic substance: you almost certainly eat some every day.

What does the evidence support?

The most-searched use is cold sores (herpes simplex). The theory is that lysine may interfere with arginine, an amino acid the cold sore virus uses to replicate. In practice the evidence is mixed and not conclusive.

A Cochrane systematic review of measures to prevent recurrent cold sores found no evidence that lysine works for prevention. Some smaller studies have hinted at a possible benefit at higher doses, while lower doses appear ineffective, but the overall picture does not support lysine as a proven treatment.

If you get frequent or severe cold sores, a pharmacist or doctor can advise on treatments with better evidence behind them, such as antiviral creams or tablets.

What foods contain lysine?

Lysine is found in protein-rich foods, so a varied diet usually supplies enough.

Good sources include:

• Meat, poultry and fish
• Eggs and dairy products such as milk, cheese and yoghurt
• Soya and soya products
• Beans, lentils and other legumes
• Some nuts and seeds

People eating a balanced diet with adequate protein are very unlikely to be short of lysine. Those following a vegan diet who eat few beans or soya foods, or anyone with a very low protein intake, may get less — combining a range of plant proteins helps.

Does lysine make you gain weight?

There is no good evidence that lysine causes weight gain. It is an amino acid, not a known driver of weight gain, and researchers note that the relationship between lysine supplementation and body weight has not been clearly established.

Like any nutrient, lysine contributes a small number of calories, but a typical supplement dose is tiny compared with your overall diet. If you're concerned about weight, the things that matter are total calories, overall diet pattern and activity — not a single amino acid.

Is lysine safe, and who should be cautious?

For most people, lysine taken by mouth at typical doses is generally considered safe for short to medium-term use. That said, there are some cautions:

• High doses may cause stomach pain, diarrhoea or other digestive upset.
• Kidney problems — there are rare reports linking lysine supplements to kidney issues, so anyone with kidney disease should check with a doctor first.
• Interactions — lysine can affect calcium handling, so be cautious if you take calcium supplements or have related conditions; discuss with a pharmacist.
• Pregnancy and breastfeeding — there isn't enough safety data, so speak to a doctor before using lysine supplements.

A supplement is not a substitute for a balanced diet or medical care. If you're considering lysine for a specific health concern, talk it through with a healthcare professional first.

You may also find our guide to zinc useful, as it's another nutrient people often look into for immune and skin health.

This page provides general, evidence-based information. It is not a substitute for advice from a qualified healthcare professional.