The amino acidlysine has been reported to reduce the recurrence rate of herpes simplex infections in both preliminary^{1, 2} and double-blind trials.^{3, 4} The amount used in these studies was usually 1 to 3 grams per day, although some people received as little as 312 mg per day. In one double-blind trial, lysine supplementation (1,200 mg per day) failed to prevent recurrences better than placebo.⁵ However, the results of that study may have been skewed by a large number of dropouts in the placebo group who fared poorly but were not included in the analysis.

When lysine has been used for acute outbreaks, the results have been mixed. In a preliminary study, 390 mg of lysine taken at the first sign of a herpes outbreak resulted in rapid resolution of the cold sores in all cases.⁶ However, in a double-blind study, supplementing with 1 gram of lysine per day for five days did not increase the healing rate of the cold sores.⁷

Most research on lysine has been done on people with cold sores or on groups that include both cold sores and genital herpes sufferers. However, some evidence exists that supplemental lysine may be effective in the prevention and treatment of genital herpes. In one preliminary survey, 81% of people with HSV infections (including genital herpes) reported lysine was effective for reducing recurrences and shortening healing time in amounts averaging about 1,000 mg per day.⁸ A small double-blind trial of people with oral and genital herpes examined the effects of 1,248 mg or 624 mg of lysine daily versus placebo.⁹ The study found that 1,248 mg per day of lysine, but not the lower dose, was effective in reducing the recurrence rate of herpes outbreaks by 57% to 65%, while neither dose helped reduce the healing time. Another small double-blind trial found that using 3,000 mg per day of lysine in divided doses led to a decrease in severity of symptoms and a reduction in healing time of both oral and genital herpes.¹⁰ One preliminary report found no benefit of lysine for a group of patients with either oral or genital herpes.¹¹

Because shingles is caused by a herpes virus, some doctors believe that lysine supplementation could help people with the condition, since lysine inhibits replication of herpes simplex, a related virus. However, lysine has not been shown to inhibit Varicella zoster, nor has it been shown to provide any benefit for people with shingles outbreaks. Therefore, its use in this condition remains speculative.