Lysine is one of nine essential amino acids — meaning your body cannot produce it and you must get it from food or supplements. It plays a foundational role in building proteins, repairing tissue, and producing enzymes, hormones, and antibodies. Without enough lysine, the body struggles to maintain muscle mass, heal wounds, and absorb calcium properly.

Lysine works in part by competing with arginine, another amino acid that the herpes virus depends on to replicate — so higher lysine levels effectively slow the virus down. It also acts as a precursor to carnitine, a compound that helps convert fatty acids into energy, and it supports collagen formation by helping stabilize the proline and glycine structure that holds connective tissue together. In the context of calcium and bone health, lysine appears to enhance intestinal calcium absorption and reduce urinary calcium loss.

Capsules and tablets are the most convenient for daily prevention use. Powder form allows flexible dosing and mixes easily into water or smoothies. Topical creams are sometimes used directly on cold sores, though oral supplementation has more clinical backing. L-Lysine HCl is the standard supplement form found in most products.

• Cold sores and herpes outbreaks (HSV-1 and HSV-2)
• Canker sores (mouth ulcers)
• Muscle recovery and lean mass
• Bone density and calcium absorption
• Anxiety and stress
• Wound healing and skin repair
• Low energy and fatigue
• Immune support

• Recurrent cold sore outbreaks (HSV-1) with daily maintenance dosing
• Calcium loss and bone thinning when dietary lysine is inadequate
• Carnitine deficiency in populations with poor protein intake

• Meat (beef, chicken, turkey) — Animal proteins are the richest source of lysine — a single serving of beef or chicken can deliver 2,000–3,000 mg, well above typical daily needs.
• Fish (tuna, salmon, cod) — Fish is an excellent lysine source and offers the added benefit of omega-3 fatty acids alongside high-quality complete protein.
• Dairy (Greek yogurt, cottage cheese, milk) — Dairy products are lysine-dense and particularly useful for vegetarians — Greek yogurt and cottage cheese deliver impressive amounts per serving.
• Legumes (lentils, black beans, chickpeas) — Legumes are the best plant-based source of lysine, making them essential for vegans and vegetarians who may otherwise fall short.
• Eggs — Eggs provide well-absorbed lysine along with all other essential amino acids, making them a practical everyday source for most diets.

• Nausea or stomach cramps at high doses (above 5,000 mg/day)
• Diarrhea with very high doses
• Rare reports of abdominal pain
• Theoretically may worsen kidney function at sustained very high doses — use caution with existing kidney disease
• May compete with arginine absorption, which could theoretically affect nitric oxide levels at very high intakes

• Recommended daily: Cold sore prevention: 1,000–3,000 mg/day; Active outbreak: 3,000 mg/day in divided doses; General supplementation: 500–1,000 mg/day
• Best time of day: Can be taken any time of day; dividing doses morning and evening improves tolerability at higher amounts
• With or without food: Can be taken with or without food; taking with food reduces the chance of stomach upset at higher doses
• Time to effect: Cold sore prevention: 2–4 weeks; Active outbreak: 3–5 days; Anxiety/stress effects: 2–4 weeks
• How long to take it: Daily maintenance for cold sore prevention is well-supported long-term. No cycling required for most uses. If using for acute outbreak, continue at higher dose through resolution then reduce to maintenance dose.