L-lysine first attracted research attention back in 1968 when scientists wondered if it could help with cold sores caused by herpes simplex virus. Since then, it’s been studied for various conditions, though the evidence remains mixed for most uses.

This article looks at what the clinical research actually shows about L-lysine supplementation, including where the evidence is stronger and where it falls short.

What is L-lysine?

L-lysine is one of nine essential amino acids, meaning your body cannot make it and must obtain it from food. As an amino acid, it’s a building block for proteins and is involved in several bodily processes.

Your body uses lysine to produce L-carnitine, a compound that helps convert fatty acids into energy. It also plays a role in calcium absorption and helps form collagen, which is needed for skin, bones, and connective tissues.

Most people get adequate lysine from their diet if they eat protein-rich foods. Deficiency is rare in developed countries but can occur in people who follow very restrictive diets or have certain medical conditions.

What are the benefits of L-lysine?

1. L-lysine may reduce cold sore recurrence

Cold sores are caused by herpes simplex virus type 1 (HSV-1), which lies dormant in nerve cells and reactivates periodically, causing painful blisters around the mouth. This is where L-lysine has its strongest theoretical basis.

The idea behind lysine supplementation is that lysine competes with another amino acid, arginine, which the virus needs to replicate. By increasing lysine intake while reducing arginine, the theory goes, you might suppress viral replication.

What the research shows:

A double-blind trial from 1987 tested 1,000 mg of L-lysine three times daily in 52 people with recurrent cold sores over six months. The lysine group had fewer outbreaks, less severe symptoms, and faster healing times compared to placebo [1].

Another 12-month study found that 1,000 mg daily reduced the number of lesions, and interestingly, people with higher blood concentrations of lysine (above 165 nmol/ml) had fewer outbreaks [2].

However, a 12-week study in 65 people found no benefit from 1,000 mg daily for recurrence rates or healing time [3].

My honest assessment: The evidence is inconsistent. Two positive studies versus one negative, all with small sample sizes. If you get frequent cold sores and want to try lysine, it’s relatively low-risk, but don’t expect dramatic results. Taking 1-3 grams daily during an outbreak or as prevention seems to be the typical approach used in studies.

2. L-lysine does not appear to help feline herpesvirus (in cats)

I’m including this because it’s a common claim in pet health circles, and the evidence goes against it.

Feline herpesvirus-1 causes respiratory infections in cats, with symptoms including discharge from eyes and nose, sneezing, and fever. Lysine supplementation became popular based on the same anti-viral logic as for human cold sores.

However, a systematic review concluded that lysine is not helpful for preventing or treating feline herpesvirus infection. Some reports even suggested lysine supplementation might increase infection frequency and disease severity [4].

If your cat has herpes-related eye or respiratory problems, lysine supplements are probably a waste of money. Talk to your vet about actually effective treatments.

3. L-lysine may have a role in schizophrenia treatment

This one caught my attention because it’s unexpected. Schizophrenia is a chronic mental illness affecting about 1% of the population, with symptoms including delusions, hallucinations, and cognitive problems.

A small crossover study tested high-dose lysine as an add-on to standard antipsychotic medications in 10 patients. The combination showed improvements in positive and negative symptoms on the PANSS scale, particularly for delusions and problem-solving abilities [5].

The catch: This was a single study with only 10 participants. That’s far too small to draw conclusions from, and I haven’t found larger follow-up studies. This remains highly preliminary and shouldn’t influence treatment decisions.

4. L-lysine may help with blood sugar control

The connection between lysine and blood sugar involves advanced glycation end products (AGEs). These are compounds formed when proteins or fats combine with sugars, and they accumulate faster in people with diabetes. AGEs contribute to diabetic complications including kidney disease and neuropathy.

A controlled study in people with type 2 diabetes found that lysine supplementation reduced serum concentrations of these harmful AGEs compared to the control group [6].

A smaller study in 13 healthy adults showed that lysine intake caused a modest decrease in blood sugar alongside increases in glucagon and insulin concentrations [7].

My take: The AGE-reduction finding is more interesting than the blood sugar effect itself. Reducing AGE formation could theoretically help prevent long-term diabetic complications, but this needs much more research. I wouldn’t take lysine expecting meaningful blood sugar improvements. If you have diabetes or related bladder issues, focus on proven treatments first.

What are the side effects of L-lysine?

At typical supplement doses, L-lysine is generally well tolerated. Side effects that have been reported include:

Diarrhoea
Nausea
Abdominal pain or cramps

These tend to be dose-related and improve when the dose is reduced.

A more serious concern involves kidney damage with long-term overdose. There are case reports of Fanconi’s syndrome and tubulointerstitial nephritis in people taking excessive amounts of lysine over extended periods. Warning signs include pain between the buttocks and ribs, decreased urination, nausea and vomiting, and bloody stools.

If you have existing kidney problems or are prone to kidney stones, be cautious with lysine supplementation and consult your doctor.

Safety precautions (9 contraindications)

Pregnancy and breastfeeding: Don’t use lysine supplements during pregnancy or while breastfeeding due to insufficient safety data.
Calcium supplements: Lysine increases calcium absorption. If you’re already taking calcium tablets, adding lysine could theoretically lead to excessive calcium levels.
Arginine supplements: Lysine and arginine use the same transport molecules in the gut. Taking both together reduces absorption of each.
Gallstones or high cholesterol: Excessive lysine intake may increase cholesterol levels, potentially increasing gallstone risk.
Hyperlysinemia or hyperlysinuria: People with these rare metabolic disorders should avoid supplementation entirely.
Impaired liver or kidney function: Avoid use due to unknown safety in these populations.
Aminoglycoside antibiotics: Lysine may affect how these drugs work. These include streptomycin, kanamycin, tobramycin, gentamicin, neomycin, amikacin, and netilmicin.
Gastrointestinal motility drugs: May interact with prucalopride and tegaserod.
Lysinuric protein intolerance: This rare inherited condition impairs amino acid transport. Lysine supplementation can cause diarrhoea and stomach cramps in affected individuals.

What is the recommended dose of L-lysine?

The basic requirement for maintaining health is approximately 30 mg per kilogram of body weight daily. For a 70 kg adult, this works out to about 2.1 grams from food sources.

For cold sore prevention or treatment, studies have used 1-3 grams daily in divided doses. Some practitioners recommend higher doses (3 grams) during an active outbreak and lower maintenance doses (1 gram) for prevention.

Most supplements come in 500 mg or 1,000 mg tablets. Taking with food may reduce stomach upset.

Foods rich in L-lysine

If you eat meat, dairy, or eggs regularly, you’re likely getting plenty of lysine. Here are some of the best food sources (content per 100 grams):

Food	Lysine content
Lean beef and lamb	3,582 mg
Parmesan cheese	3,306 mg
Chicken	3,110 mg
Pork	2,757 mg
Soybeans and tofu	2,634 mg
Tuna	2,590 mg
Shrimp	2,172 mg
Pumpkin seeds	1,386 mg
Eggs	912 mg

Vegetarians and vegans can get adequate lysine from soy products, legumes, and seeds, though they may need to be more intentional about including these foods regularly.

The bottom line

L-lysine is an essential amino acid that your body needs, and most people get enough from food. As a supplement, it has the most evidence (though still mixed) for reducing cold sore frequency and severity. The research on blood sugar, schizophrenia, and other conditions is too preliminary to guide decisions.

If you’re considering lysine supplementation, keep expectations modest and be aware of the contraindications, particularly if you take other medications or have kidney issues.

References

Griffith RS, Walsh DE, Myrmel KH, Thompson RW, Behforooz A. Success of L-lysine therapy in frequently recurrent herpes simplex infection. Treatment and prophylaxis. Dermatologica. 1987;175(4):183-90. PubMed
Thein DJ, Hurt WC. Lysine as a prophylactic agent in the treatment of recurrent herpes simplex labialis. Oral Surg Oral Med Oral Pathol. 1984 Dec;58(6):659-66. PubMed
Milman N, Scheibel J, Jessen O. Lysine prophylaxis in recurrent herpes simplex labialis: a double-blind, controlled crossover study. Acta Derm Venereol. 1980;60(1):85-7. PubMed
Bol S, Bunnik EM. Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review. BMC Vet Res. 2015;11:284. PubMed
Wass C, Klamer D, Katsarogiannis E, Pålsson E, Svensson L, Fejgin K, Bogren IB, Engel JA, Rembeck B. L-lysine as adjunctive treatment in patients with schizophrenia: a single-blinded, randomized, cross-over pilot study. BMC Med. 2011 Apr 12;9:40. PubMed
Mirmiranpour H, Khaghani S, Bathaie SZ, Nakhjavani M, Kebriaeezadeh A, Ebadi M, Gerayesh-Nejad S, Zangooei M. The Preventive Effect of L-Lysine on Lysozyme Glycation in Type 2 Diabetes. Acta Med Iran. 2016 Jan;54(1):24-31. PubMed
Kalogeropoulou D, Lafave L, Schweim K, Gannon MC, Nuttall FQ. Lysine ingestion markedly attenuates the glucose response to ingested glucose without a change in insulin response. Am J Clin Nutr. 2009 Aug;90(2):314-20. PubMed
NHS. Amino acids. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/
Mayo Clinic. Lysine. Available at: https://www.mayoclinic.org/drugs-supplements-lysine/art-20366421