Oral valacyclovir is better absorbed than oral acyclovir, increasing acyclovir bioavailability three-to fivefold. This provides the opportunity to explore whether high systemic acyclovir concentrations are effective in the treatment of cold sores (herpes labialis). Two randomized, double-blind, placebo-controlled studies were conducted. Subjects were provided with 2 g of valacyclovir twice daily for 1 day (1-day treatment), 2 g of valacyclovir twice daily for 1 day and then 1 g of valacyclovir twice daily for 1 day (2-day treatment), or a matching placebo and instructed to initiate treatment upon the first symptoms of a cold sore. In study 1, the median duration of the episode (primary endpoint) was reduced by 1.0 day (P ؍ 0.001) with 1-day treatment and 0.5 days (P ؍ 0.009) with 2-day treatment compared to placebo. Similarly, the mean duration of the episode was statistically significantly reduced by 1.1 days with 1-day treatment and 0.7 days with 2-day treatment compared to placebo. The proportion of subjects in whom cold sore lesion development was prevented and/or blocked was increased by 6.4% (P ؍ 0.096) with 1-day treatment and 8.5% (P ؍ 0.061) with 2-day treatment compared to placebo. The time to lesion healing and time to cessation of pain and/or discomfort were statistically significantly reduced with valacyclovir compared to placebo. In study 2, results similar to those in study 1 were obtained. AEs were similar across treatment groups. These studies provide evidence supporting a simple, 1-day valacyclovir treatment regimen for cold sores that is safe and effective. The 1-day valacyclovir regimen offers patients a unique and convenient dosing alternative compared to available topical therapies.Herpes labialis, or cold sores, are caused by herpes simplex virus type 1 (HSV-1) and can result in significant irritation, pain, discomfort, and worry (18,25). The infection is most often acquired in childhood, but the incidence increases with age (2, 5, 12, 18). Up to 90% of persons over the age of 50, depending on race, sex, and country, would test seropositive for HSV-1, and an estimated 20 to 40% of adults experience cold sore outbreaks (5,12,18,25).Until now, most treatments have consisted of creams that must be applied to affected areas multiple times a day for several days (6,13,15,19,20,23). There have been two previous clinical trials of oral antiviral therapy for episodic treatment of herpes labialis. In a double-blind, placebo-controlled trial involving 210 subjects with recurrent cold sores, it appeared that less time was required for the loss of hard crust for subjects taking 200 mg of oral acyclovir 5 times daily for 5 days (14). This difference as well as other measures of clinical healing was not statistically significant. Spruance et al. evaluated a regimen of 400 mg of oral acyclovir 5 times daily for 5 days in comparison with placebo (21). Overall in this study, acyclovir did not affect the duration of the episode, mean maximum lesion size, or the development of lesions, but a subset o...