Lysine for Cold Sores: One Trial at 3 g a Day Looked Good, but Lower Doses Mostly Failed
Lysine is the supplement most often taken to keep cold sores away. The placebo-controlled trials are small and date from the 1980s or earlier: the one using 3 g a day reported fewer outbreaks, while trials at about 1 g a day mostly found nothing, and a Cochrane review found no evidence it works. Here is what each trial found, who paid for it, and who should be careful.
By Kymata Health Editorial Published Last checked
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Key takeaways
From 9 sources- The most positive trial gave 1,000 mg of L-lysine three times a day (3 g) for 6 months; the 27 people on lysine who finished had on average 2.4 fewer herpes outbreaks than the 25 on placebo, with milder symptoms1.
- At lower doses results were weak: 1,248 mg a day showed some sign of fewer recurrences but 624 mg did not2, 1,000 mg a day had no overall effect on recurrence3, and 1,200 mg a day showed no substantial benefit4.
- A 2015 Cochrane review of 32 trials of cold-sore prevention found no evidence of efficacy for lysine, while long-term prescription antiviral tablets had a small benefit5.
- In a safety review of 71 human studies, side effects were mainly stomach upset, nausea and diarrhea, and the authors set a provisional no-adverse-effect level of 6,000 mg a day7.
- None of these trials is recent or large, so the honest answer is that lysine is unproven: possibly helpful at about 3 g a day on the strength of one trial, and no better than placebo in most trials at lower doses1346.
What this article covers
1 evidence profileThe evidence, graded
Pulled from the trial tables in our evidence profiles. Grades follow our evidence-grading policy.
Graded findings
4 rows- DGrade DRecurrent herpes simplex: outbreaks, severity and time to clearImproved The lysine group had an average of 2.4 fewer outbreaks, milder symptoms and faster healing than placebo.Lysine · RCT · n=52 · 1987 · PMID 3115841
- DGrade DRecurrent herpes simplex: recurrence rateMixed 1,248 mg/day showed evidence of fewer recurrences; 624 mg/day did not. Neither dose shortened healing time.Lysine · Crossover RCT · n=41 · 1984 · PMID 6435961
- DGrade DRecurrent cold sores (herpes labialis)Mixed No overall effect on recurrence rate, healing or lesion appearance, though more people were recurrence-free on lysine than on placebo (p = 0.05).Lysine · Crossover RCT · n=65 · 1980 · PMID 6153847
- DGrade DRecurrent herpes simplex: frequency, duration and severityNo effect No substantial benefit for outbreaks in progress or for preventing recurrences.Lysine · RCT · n=21 · 1984 · PMID 6419679
Where these trials agree: Lysine and cold sores, counted row by row in the Consensus finder.
Why people take lysine for cold sores
Cold sores are caused by the herpes simplex virus, which stays in the body and flares up from time to time. Lysine is an amino acid found in protein foods, and in cell cultures it slows the multiplication of the virus, which is where the idea came from3.
A 2017 review noted that using lysine to stop herpes recurrences is not widely recognised by doctors, and set out to summarise the trials6. The human trials below are the real test.
Have a bottle? Check its lysine dose against the trials in Label Lens.
The trial that found a benefit used 3 g a day
In a double-blind, placebo-controlled trial led from Indiana University School of Medicine across several centres, people with frequently recurring herpes took L-lysine tablets (1,000 mg three times a day) or placebo for 6 months1. Of those who finished, 27 were on lysine and 25 on placebo1.
The lysine group averaged 2.4 fewer outbreaks, and their symptoms were milder and cleared faster than on placebo (all p < 0.05)1. The abstract does not describe how people were randomised or how many dropped out, and its PubMed record lists no funder1.
Trials at about 1 g a day mostly found nothing
A crossover trial in 41 adults with normal immune systems compared 624 mg and 1,248 mg of L-lysine a day with placebo2. The higher dose showed evidence of fewer recurrences, the lower dose did not, and neither dose shortened the time for sores to clear2.
A double-blind crossover trial gave 65 people 1,000 mg a day for 12 weeks and then placebo for 12 weeks3. Overall, lysine had no effect on how often cold sores came back, how fast they cleared or how bad they looked, although more people were recurrence-free on lysine than on placebo (p = 0.05)3.
A placebo-controlled trial in 21 people with frequent outbreaks used 400 mg three times a day and could not detect any substantial benefit, either for an outbreak in progress or for stopping recurrences4. The authors concluded that lysine is unlikely to help most people with frequent recurrences4.
What the reviews concluded
The 2015 Cochrane review covered 32 randomised trials with 2,640 people and 19 different approaches to stopping cold sores from coming back5. It found no evidence of efficacy for lysine5. By contrast, long-term prescription antiviral tablets did reduce recurrences, though the benefit was small: in one trial, aciclovir cut clinical recurrences from 1.80 to 0.85 per person over 4 months5. The review authors declared nothing to disclose5.
The 2017 review read the same trials more generously: it judged lysine ineffective below 1 g a day without a low-arginine diet, said doses above 3 g a day appear to improve how people experience the disease, and called for longer controlled trials above 1.2 g a day6. Its PubMed record lists no funder6.
For other supplements tried for immune support, see our explainers on zinc lozenges and echinacea. The lysine page grades each lysine finding.
Who should be careful
Check with a doctor or pharmacist before taking lysine if any of these apply:
- Kidney disease: a published case report describes a 44-year-old woman who developed Fanconi syndrome and kidney inflammation linked to lysine, which progressed to chronic kidney failure8.
- Heart or gallbladder disease: the 2017 review advises warning people with these conditions about theoretical risks of lysine supplements6.
- Taking calcium supplements or medicines that affect calcium: in small human studies, lysine increased calcium absorption from the gut and reduced calcium lost in urine9. We found no published interaction studies between lysine and prescription drugs, which means the risk is unknown rather than shown to be zero, so tell your pharmacist you take it.
- Already on prescription antivirals for cold sores: long-term antiviral tablets have trial evidence of a small benefit and lysine does not5, so any change should be made with the prescriber.
- High doses: the safety review, funded by the International Council on Amino Acid Science, set a provisional no-adverse-effect level of 6,000 mg a day based on stomach symptoms7, so the 3 g a day used in the positive trial1 is below it, but higher amounts are less studied.
- Weakened immune system, pregnancy or children: the trials enrolled adults, one of them only people with normal immune systems2, and none reports separate results for pregnancy1234, so these groups need a clinician's advice.
Quick answers
Common questions
4 answersHow much lysine was used in the trials?
Can lysine shorten a cold sore that has already started?
Is lysine as good as antiviral medicine?
No trial in these reviews compared them head to head, but Cochrane found trial evidence of a small benefit for long-term antiviral tablets and no evidence of efficacy for lysine5.
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Where to buy Lysine
Products from brands we partner with that contain Lysine. Check the amount per serving against the doses studied, and the label for other ingredients.
Lysine products from partner brands
3 products Paid links · we earn a commission- Quick pickHerbsProNOW Foods L-Lysine, 500 mg, 100 Tabs
1 tablet: 500 mg L-lysine (from L-lysine HCl)
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6 capsules: 3,000 mg L-lysine (500 mg per capsule); 30 servings
$27.00one-time$22.95subscribeChecked Oct 11, 2026 - Dr. TobiasDr. Tobias L-Lysine 500 mg (100 tablets)
1 tablet: 500 mg L-lysine (as L-lysine HCl); label directions 1 to 3 tablets daily
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References
9 sources. Numbers in the text link here.
Sources cited in this article
9 sources- 1Success of L-lysine therapy in frequently recurrent herpes simplex infection. Treatment and prophylaxisDermatologica, 1987 · PMID 3115841
- 2Treatment of recurrent herpes simplex infections with L-lysine monohydrochlorideCutis, 1984 · PMID 6435961
- 3Lysine prophylaxis in recurrent herpes simplex labialis: a double-blind, controlled crossover studyActa Derm Venereol, 1980 · PMID 6153847
- 4Failure of lysine in frequently recurrent herpes simplex infection. Treatment and prophylaxisArch Dermatol, 1984 · PMID 6419679
- 5Interventions for prevention of herpes simplex labialis (cold sores on the lips)Cochrane Database Syst Rev, 2015 · PMID 26252373
- 6Lysine for Herpes Simplex Prophylaxis: A Review of the EvidenceIntegr Med (Encinitas), 2017 · PMID 30881246
- 7Safety assessment of L-lysine oral intake: a systematic reviewAmino Acids, 2019 · PMID 30661148
- 8Fanconi's syndrome and tubulointerstitial nephritis in association with L-lysine ingestionAm J Kidney Dis, 1996 · PMID 8840955
- 9Dietary L-lysine and calcium metabolism in humansNutrition, 1992 · PMID 1486246
Lysine and other dietary supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Talk to your clinician or pharmacist before starting a supplement, especially if you take prescription medicines. Product links on this page are paid partner links.