L-Lysine for Canker Sores
L-lysine is NOT effective for canker sores (aphthous ulcers) and should not be used for this condition. The evidence supporting lysine is exclusively for herpes simplex virus infections (cold sores/herpes labialis), which are completely different lesions caused by a viral pathogen, whereas canker sores are non-infectious inflammatory ulcers.
Critical Distinction Between Conditions
Canker sores and cold sores are entirely different entities:
Canker sores (aphthous ulcers) are non-infectious inflammatory lesions that occur inside the mouth on non-keratinized mucosa (inner cheeks, tongue, soft palate). They are NOT caused by herpes simplex virus 1.
Cold sores (herpes labialis) are viral infections caused by herpes simplex virus that typically occur on the lips and perioral skin. These are the lesions for which lysine has been studied 1, 2.
Evidence for Lysine in Herpes Simplex (Not Canker Sores)
The lysine research applies ONLY to herpes simplex infections:
A 1983 survey of 1,543 subjects with herpes infections (cold sores and genital herpes—NOT canker sores) reported that 84% felt lysine prevented recurrence, though this was a subjective questionnaire study without placebo control 2.
A 1984 randomized controlled trial found that 1,248 mg daily of L-lysine monohydrochloride decreased recurrence rates of herpes simplex attacks, but 624 mg daily was ineffective. Neither dose shortened healing time 3.
However, another 1984 double-blind, placebo-controlled trial of 400 mg three times daily found NO substantial benefit for herpes simplex treatment or prophylaxis 4.
The evidence for lysine even in herpes simplex is mixed and of limited quality, with conflicting results from small studies conducted decades ago.
Why This Matters Clinically
Patients frequently confuse canker sores with cold sores, leading to inappropriate self-treatment. If a patient presents asking about lysine for "canker sores," you must:
Verify the actual diagnosis by asking where the lesions occur (inside mouth = likely canker sores; lips/perioral = likely cold sores) 1.
Clarify that lysine has zero evidence for canker sores and the studies cited by patients or found online refer exclusively to herpes simplex 2, 3, 4.
Redirect treatment appropriately: For true canker sores, management focuses on topical analgesics, antiseptics, and addressing underlying triggers (trauma, stress, nutritional deficiencies), not antiviral or lysine therapy 1.
Bottom Line
Do not recommend lysine for canker sores—there is no evidence base, no biological rationale, and it represents a fundamental misunderstanding of the condition. The lysine literature pertains exclusively to herpes simplex virus infections, which are diagnostically and therapeutically distinct from aphthous ulcers 1, 2, 3, 4.