L-lysine is an essential amino acid your body cannot make on its own, so you need to get it from food or supplements. It plays a role in protein synthesis, collagen formation, calcium absorption, and the production of carnitine, a compound your body uses to convert fat into energy. Most people get enough lysine from a balanced diet, but supplements are commonly used for cold sore prevention, stress-related anxiety, and bone support.
Cold Sore Prevention
The most popular reason people take L-lysine is to reduce outbreaks of cold sores caused by the herpes simplex virus (HSV-1). The virus needs arginine, another amino acid, to replicate and build its viral proteins. Lysine works by directly competing with arginine, blocking the virus from using it efficiently. It also promotes the breakdown of arginine in the body, further starving the virus of what it needs to multiply.
The evidence for prevention is stronger than for treatment. Two double-blind, placebo-controlled trials found that doses of 1,000 to 1,248 mg per day significantly reduced how often cold sores came back. One randomized trial showed an even clearer effect at 3,000 mg daily. However, once a cold sore has already appeared, lysine doesn’t seem to help much. Two randomized controlled trials found no significant benefit from lysine supplements for treating active outbreaks, even at doses above 1,000 mg per day. In short, lysine is better at keeping cold sores away than at healing one that’s already formed.
Stress and Anxiety
A double-blind, placebo-controlled study of 108 healthy Japanese adults found that taking L-lysine (2.64 g per day) combined with L-arginine (2.64 g per day) for one week reduced baseline levels of cortisol, the body’s primary stress hormone. The combination also lowered a salivary marker tied to the “fight or flight” nervous system in male subjects. Participants with high self-reported stress and anxiety saw the most benefit.
It’s worth noting that this research used lysine and arginine together, not lysine alone. The combination appears to be what moves the needle on stress hormones. If anxiety reduction is your goal, taking lysine by itself may not produce the same effect.
Calcium Absorption and Bone Health
Lysine improves how your body handles calcium in two ways: it increases how much calcium you absorb from food in your intestines, and it helps your kidneys hold onto calcium instead of flushing it out in urine. In a study of 45 patients with osteoporosis, supplementing with 800 mg of L-lysine per day significantly increased intestinal calcium absorption. Two other amino acids tested at the same dose, valine and tryptophan, had no effect. Healthy subjects who took lysine also showed a blunted spike in urinary calcium after a calcium load, meaning their bodies retained more of the mineral.
This makes lysine particularly relevant for people at risk of bone loss, including postmenopausal women and older adults who may already struggle with calcium absorption.
Collagen and Tissue Repair
Your body uses lysine as a building block for collagen, the protein that gives structure to skin, tendons, cartilage, and blood vessels. Specifically, lysine residues within collagen and elastin are chemically modified by enzymes that create cross-links between protein fibers. These cross-links are what make connective tissue strong and resilient rather than loose and fragile.
After a skin injury, the enzymes responsible for cross-linking ramp up before collagen production even begins, essentially preparing the scaffolding before new tissue is laid down. This cross-linking activity increases through the later stages of inflammation and continues into tissue remodeling, helping granulation tissue mature into stable scar tissue. Without adequate lysine, this entire process slows down. People recovering from surgery or burns sometimes supplement for this reason, though specific wound-healing dosages haven’t been standardized in clinical trials.
Energy Production and Fat Metabolism
Lysine is one of two amino acids (along with methionine) your body uses to manufacture carnitine. Carnitine acts as a shuttle system inside your cells: it picks up long-chain fatty acids and carries them into the mitochondria, where they’re burned for energy. Without enough carnitine, your cells can’t efficiently use stored fat as fuel.
This has made lysine appealing in fitness circles, but the connection to athletic performance is indirect. Some small studies on carnitine supplementation have shown modest improvements in endurance and energy production during exercise. However, a well-controlled trial of 32 healthy adults found that supplemental carnitine for eight weeks did not improve aerobic or anaerobic performance. Taking lysine to boost carnitine levels is a reasonable biological strategy, but expecting noticeable gains in the gym would be getting ahead of the evidence.
Food Sources
Most people eating a varied diet get plenty of lysine without trying. High-lysine foods include poultry, fish, shrimp, shellfish, pork, beef, eggs, soy, beans, lentils, nuts, and seeds. Animal proteins tend to be the richest sources. Vegetarians and vegans can meet their needs through legumes and soy products, though they may need to be more intentional about it. Lysine is the amino acid most commonly lacking in grain-heavy diets, since wheat, rice, and corn are relatively low in it.
Dosage and Safety
For cold sore prevention, the best-supported doses in controlled studies range from 1,000 to 3,000 mg per day. For calcium absorption, benefits were seen at 800 mg per day. The anxiety research used a higher dose of 2,640 mg per day, combined with arginine.
At typical supplement doses, lysine is generally well tolerated. The most common side effects are digestive: nausea, stomach cramps, or diarrhea. The more serious concern involves kidney health. Animal studies have shown that very high doses of lysine can cause acute kidney injury resembling what’s seen in human “acute tubular necrosis,” with damage to individual kidney cells. These were doses far above what supplements provide, but people with existing kidney problems should be cautious with high-dose or long-term use.
One specific drug interaction to be aware of: lysine taken alongside aminoglycoside antibiotics (such as gentamicin, neomycin, or streptomycin) may increase the risk of kidney damage, since both substances stress the kidneys through similar pathways.