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L-lysine is one of the nine essential amino acids, the building blocks of protein that your body cannot make and must get from food. As a supplement it is mainly known for one use: people with recurring cold sores take it hoping for fewer outbreaks. The research on that is mixed. One small 1987 trial reported fewer and milder outbreaks, but a 2015 Cochrane review of prevention studies found no evidence that lysine works. Most people who eat enough protein already get plenty of lysine from their diet. Here is what lysine does, what the studies tested, and the safety points worth knowing.
Peptide Labs sells one amino acid formula that lists L-lysine among several ingredients, but this article does not recommend any product. It is general information, not medical advice.
Key Takeaways
- Lysine is an essential amino acid: your body needs it to build protein and cannot make it.
- People who eat enough protein from a varied diet usually get enough lysine from food.
- A small 1987 trial reported fewer cold sore outbreaks, but a 2015 Cochrane review found no evidence lysine helps prevent them.
- Safety research on lysine supplements is limited, and a case report linked them to serious kidney injury.
- Anyone with kidney disease, who is pregnant or breastfeeding, or who takes calcium supplements should ask a clinician first.
What is L-lysine?
Amino acids link together to form proteins, which your body uses to break down food, grow and repair tissue. MedlinePlus, the National Library of Medicine’s consumer site, sorts them into three groups. Nonessential amino acids can be made by the body. Essential amino acids cannot, so they must come from food, and lysine is one of the nine. The others are histidine, isoleucine, leucine, methionine, phenylalanine, threonine, tryptophan and valine.
The “L” in L-lysine refers to the form of the molecule the body uses, and it is the form found in food protein and in supplements. On a Supplement Facts panel you will often see “L-lysine HCl” (hydrochloride), a common salt form.
Lysine is part of protein-rich foods such as meat, fish, eggs, dairy, beans and soy. MedlinePlus notes that a diet built on a single plant food is not adequate, but you no longer need to pair proteins at every meal; what matters is variety across the whole day.
Lysine benefits: what the research shows
Searches for lysine benefits usually lead to the cold sore question, and that is where almost all of the human research sits. A few small studies have looked at other areas, such as calcium absorption, but none is strong enough to show a clear benefit for healthy people. This table sums up the studies people cite most often.
| Study | Who took part | What it found | Limits |
|---|---|---|---|
| Griffith 1987, US | 52 adults with frequent herpes outbreaks who finished the trial | Fewer outbreaks, milder symptoms and faster healing than placebo | Small, older, single trial |
| Chi 2015 Cochrane review | 32 trials, 2,640 people, 19 prevention approaches | No evidence that lysine helps keep cold sores away | Few lysine trials, mostly low-quality evidence |
| Civitelli 1992, Italy | Small groups of healthy and osteoporotic women | Lysine raised calcium absorption in a lab-style test | Short, tiny, no bone outcomes measured |
| Lo 1996 case report | One 44-year-old woman | Kidney injury linked to oral lysine | A single case cannot prove cause |
L-lysine for cold sores: the evidence
Cold sores on the lips are caused by the herpes simplex virus, which stays in the body and can flare up again. The idea behind lysine comes from lab work suggesting the virus relies on another amino acid, arginine, and that extra lysine might compete with it. That theory has not been confirmed in people.
The trial most often quoted is a 1987 double-blind, placebo-controlled study led by Richard Griffith at Indiana University. Of the people who finished six months, 27 took lysine and 25 took a placebo. The lysine group averaged 2.4 fewer outbreaks, and their symptoms were milder and healed faster. It was a small trial, and later studies did not settle the question.
In 2015 the Cochrane Collaboration, which reviews medical evidence independently, looked at 32 randomized trials of ways to prevent cold sores in 2,640 people with healthy immune systems. The authors wrote that they found no evidence of efficacy for lysine. The approach with the clearest support was long-term prescription antiviral medicine, and even there the benefit was small.
If you get frequent or severe cold sores, sores near the eyes, or outbreaks while your immune system is weakened, see a clinician or dermatologist. A supplement is not a substitute for a medical review, and prescription options exist.
Lysine side effects and safety
Lysine is a normal part of food, but safety research on lysine supplements is limited, and a concentrated single amino acid is not the same as eating protein, and a few safety points matter.
- Kidney injury. A 1996 case report in the American Journal of Kidney Diseases described a woman who developed a serious kidney disorder (Fanconi syndrome with tubulointerstitial nephritis) after taking oral lysine, which progressed to chronic kidney failure. One case cannot prove cause, but people with kidney disease should not take lysine without their nephrologist’s approval.
- Calcium. Small studies found lysine increased calcium absorption. That is not a proven bone benefit, but it is worth telling your clinician if you also take calcium supplements.
- Pregnancy and breastfeeding. There is little safety research, so ask your obstetric provider first.
- Rare genetic conditions. People with inherited disorders of amino acid metabolism follow diets set by their specialists and should not add single amino acids on their own.
Amino acid powders and capsules are dietary supplements, which FDA does not approve before sale. Our guide to third-party tested supplements explains what a testing seal does and does not tell you.
Lysine in multi-ingredient formulas
Lysine often appears inside blends rather than on its own. We carry one example: the DR Vitamin Solutions Nighttime Weight Loss capsules. Its Supplement Facts panel lists L-lysine HCl alongside L-arginine, L-ornithine, L-glutamine, bovine colostrum, ornithine alpha-ketoglutarate and glycine, in a bovine gelatin capsule. The label describes it as caffeine-free. It is not a lysine supplement, and it contains both lysine and arginine, the two amino acids in the cold sore theory. Because it includes colostrum and gelatin, it is not suitable for vegetarians or anyone avoiding milk-derived ingredients.
If you want to compare related ingredients, read our explainers on L-glutamine and colostrum. For cold-season supplements, our zinc guide covers what the research on lozenges does and does not show.
How to read a lysine label
- Check whether lysine is the only ingredient or one item in a blend, and whether amounts are disclosed or hidden in a proprietary blend.
- Look for the form, usually L-lysine HCl, and the capsule material if you avoid animal products.
- Read the warnings panel, and keep the product out of reach of children.
- Look for an independent testing seal and a lot number.
- Follow the label directions, and bring the bottle to your clinician or pharmacist if you take other medicines or have kidney problems.
Frequently Asked Questions
What does L-lysine do in the body?
Lysine is an essential amino acid. Your body uses it, along with the other amino acids, to build the proteins that make up tissue, enzymes and hormones. It must come from food because the body cannot make it.
Does L-lysine help with cold sores?
The evidence is mixed. A small 1987 trial reported fewer outbreaks, but a 2015 Cochrane review found no evidence that lysine reduces cold sore outbreaks. Frequent outbreaks are worth discussing with a clinician.
What are the main lysine side effects?
Safety research is limited. The most serious report is a single case of kidney injury linked to lysine supplements, so people with kidney disease should check with their doctor, and anyone with new symptoms should stop and call a clinician.
Do I need a lysine supplement if I eat enough protein?
Most people who eat a varied diet with enough protein get lysine from food. Supplements are not needed for general health unless a clinician has a specific reason.
Can I take lysine while pregnant?
There is little safety research in pregnancy or breastfeeding. Ask your obstetric provider before taking any single amino acid supplement.
Is lysine the same as arginine?
No. Both are amino acids, but lysine is essential and arginine can be made by the body. The cold sore theory is based on lab studies suggesting the two compete.
Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Last reviewed October 2026.
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