L-Lysine
An essential amino acid, best known as what people take for cold sores.
- Cold sores coming back
- Stress and nerves
- Bone and calcium
- Muscle and strength
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is L-Lysine?
L-lysine is an essential amino acid, which means the body cannot make it and every bit of it has to arrive in food. A diet built on wheat runs short of lysine before anything else, which is why it has long been added to flour in countries where grain is the staple. The lysine in a tub is made by fermenting bacteria, on a scale of more than a million tonnes a year, most of it going to food and animal feed. In Britain it is sold as plain L-lysine, or as lysine hydrochloride, and it is best known as what people reach for when cold sores keep coming back.
Why do people take L-Lysine?
Cold sores coming back
The thinking. Cold sores are what most people in Britain buy lysine for, and they buy it with an eye on how often the next one arrives rather than on the one they have. The idea rests on a balance between two amino acids: in cell cultures the herpes simplex virus multiplies more readily when arginine outweighs lysine, and less readily when the balance tips the other way, something reviewers call lysine–arginine antagonism. Whether a tablet shifts that balance enough to matter in a person is what the trials measured.
What research has found. Seven studies, all of them between 1978 and 1987, counted how often sores came back, how long they lasted and how bad they were, in groups of twenty to sixty-five people taking roughly 300 mg to 3 g a day. Three recorded fewer sores on lysine, one found nothing to report, two found a difference on one measure and not on others, and the largest was a questionnaire of more than 1,500 people with nothing to compare against. Dose separates them more than anything else: in the one trial that compared two amounts, the higher moved the recurrence rate and the lower did not. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
Stress and nerves
The thinking. Some people take lysine hoping to feel steadier when life is demanding. The idea came out of animal work, where animals kept on diets short of lysine were more rattled by stress than animals given enough. It was first put to people whose meals are built on wheat, because wheat is the cereal lysine runs shortest in.
What research has found. One three-month randomised trial in northern Syria gave families wheat fortified with lysine or the same wheat without it, and measured anxiety questionnaires alongside the body’s own signals of stress. Long-standing anxiety scores came down among the men, the rise in cortisol around having blood taken was smaller among the women, and a skin measure of arousal fell in the men. Few studies have measured this yet, so there is little to go on either way.
Bone and calcium
The thinking. Bone is another reason people pick lysine up, and two of its everyday jobs sit behind the idea. Reviews list taking up minerals among the things the body uses lysine for, and lysine is one of the points at which collagen is cross-linked — collagen being the protein scaffold that bone mineral is laid down on. Reviewers name the use of lysine in osteoporosis among the questions they would most like to see looked into.
What research has found. No trial in people has measured this, so there is nothing to report either way yet.
Muscle and strength
The thinking. Muscle is protein, and reviews describe lysine as a key building block of the protein the body puts together, so the idea reads simply enough: if the diet is short of lysine, what muscle can do is one of the places it might show. Researchers have mostly asked the question where the everyday diet is thin on it.
What research has found. One controlled eight-week feeding study in young men, some undernourished and some not, compared each man’s usual lysine intake with a higher one of 80 mg for every kilogram of body weight a day. Muscle strength measured about seven and a half per cent better on the higher intake, while muscle mass, insulin sensitivity and the other measures did not differ. Very little research has measured this so far, either way.
What does L-Lysine do in the body?
Lysine is a building block. The body strings amino acids together into every protein it makes, and lysine is one it cannot produce for itself, so the whole supply depends on what comes in. Reviews describe it as indispensable for putting protein together, for the cross-links that hold collagen and elastin in shape, for taking minerals up, and as one of the raw materials the body uses to make carnitine.
It is carried into cells across the membrane, and reviews describe a push and pull between lysine and arginine as they go — lysine–arginine antagonism — which is where the cold-sore idea began. Whatever the body has no use for is broken down along one main path, and how much is absorbed from a supplement is described as much like the protein in a meal. Where a diet falls short of it for a long time, reviews say the shortfall shows in connective tissue, in the blood and in children’s growth.
What do people say about taking L-Lysine?
These are anecdotes people have shared, not evidence.
What people like
- Five of the six threads read sit in one parenting forum, and almost all of the talk is about cold sores rather than anything else.
- Taking a tablet every day, for years, is the commonest routine described, and several say outbreaks became rare or stopped while they kept it up.
- Several describe swallowing much more at the first tingle, several grams over a day or two, and say a sore never arrived at all.
Common complaints
- A handful say it did nothing for them, one describing years of daily tablets with no change in how often sores came back.
- Hopes beyond cold sores, thicker hair and better nails or skin, went unmet by everyone who answered the one thread that asked about them.
- Digestive upset comes up more as a worry than as something experienced, alongside questions about what years of it might do to the kidneys.
- Several reach for prescription antiviral tablets instead, or alongside it, and rate those as the more dependable of the two when a sore starts.
How people take it
- A daily tablet of 500 mg to 1,000 mg is the routine described most often, kept up for months or years on end.
- At the first tingle most go much higher for a few days, with figures ranging from 1,500 mg a day up to 4,000 mg at once.
- A couple raise the amount during stressful patches instead, naming exam weeks, broken sleep and the week before a period rather than waiting for a sore.
- A lysine lip balm is the thing most often used alongside the tablets; zinc and vitamin C are named once, together, during an outbreak.
- Several stopped while pregnant, one after a nutritionist said it was fine and a midwife said otherwise; nobody reported clear advice either way.
Read from these public discussions:
- Lysine for coldsores (Mumsnet, 2022)
- Why are more people not talking about lysine supplements? (Mumsnet, 2023)
- Can I take l lysine for coldsore? (Mumsnet, 2021)
- Coldsores (Mumsnet, 2018)
- Cold sores (Mumsnet, 2019)
- L-Lysine for cold sores (A fitness forum, 2008)
What forms of L-Lysine are there?
| Form | What’s different | Per serving, on products we list |
|---|---|---|
| L-Lysine | Plain L-lysine; labels also name it lysine hydrochloride, the salt form of the same amino acid. | 1 g |
What else is worth knowing about L-Lysine?
- The review of the cold-sore trials noted that the amounts under a gram a day were tested without also lowering arginine in the diet, and called for longer trials above 1.2 grams a day. It also said people with heart or gallbladder disease should be told about the theoretical risks.
- Two systematic reviews pooled every trial of lysine taken by mouth they could find — 71 studies, 3,357 people — at daily amounts from under 20 mg to 17.5 grams, for a single day up to three years. The side effects recorded were mainly stomach ones, nausea, stomach ache and loose stools, no more often than in the comparison groups, and both set a provisional ceiling of 6 grams a day for healthy adults.
- Who took part is worth knowing: the cold-sore trials were small, none above sixty-five people, and all ran between 1978 and 1987; the stress trial was in Syrian families whose meals are built on wheat, and the muscle study in young men, some of them undernourished. One review also cautions that a great deal of one amino acid can unbalance the others.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (15 studies)
- Success of L-lysine therapy in frequently recurrent herpes simplex infection. Treatment and prophylaxis. (1987, other)
- Failure of lysine in frequently recurrent herpes simplex infection. Treatment and prophylaxis. (1984, randomised trial)
- Treatment of recurrent herpes simplex infections with L-lysine monohydrochloride. (1984, randomised trial)
- Lysine as a prophylactic agent in the treatment of recurrent herpes simplex labialis. (1984, other)
- Subjective response to lysine in the therapy of herpes simplex. (1983, other)
- Lysine prophylaxis in recurrent herpes simplex labialis: a double-blind, controlled crossover study. (1980, other)
- A multicentered study of lysine therapy in Herpes simplex infection. (1978, other)
- Lysine fortification reduces anxiety and lessens stress in family members in economically weak communities in Northwest Syria. (2004, randomised trial)
- The effect of a controlled 8-week metabolic ward based lysine supplementation on muscle function, insulin sensitivity and leucine kinetics in young men. (2012, randomised trial)
- Lysine for Herpes Simplex Prophylaxis: A Review of the Evidence. (2017, review)
- L-lysine dietary supplementation for childhood and adolescent growth: Promises and precautions. (2025, review)
- Lysine: Sources, Metabolism, Physiological Importance, and Use as a Supplement. (2025, review)
- Expanding lysine industry: industrial biomanufacturing of lysine and its derivatives. (2018, review)
- Comprehensive Review of L-Lysine: Chemistry, Occurrence, and Physiological Roles. (2026, review)
- Lysine and salt-sensitive hypertension. (2024, review)
How much does L-Lysine cost?
That depends on the form and the pack, so we work out the price of the same dose of every L-Lysine product we can find and rank them.