In people who have had at least one outbreak of blistering from genital herpes, the drug famciclovir sharply reduces virus shedding from the external portions of the genitalia, a new study finds. Such shedding can spread the virus between people.

Despite the apparent risk of herpes spreading during an outbreak, most new cases of genital herpes are caused by sexual contact with an infected person without visible blisters, says Peter Leone, a physician at the University of North Carolina School of Medicine in Chapel Hill. Because such silent transmission "is what drives the epidemic," he says, inhibiting shedding could prove valuable.

Famciclovir (Famvir) is a daily antiviral pill prescribed to limit herpes outbreaks. To test whether it can also stop viral shedding, researchers identified 129 men and women with genital herpes and randomly assigned half to take famciclovir and half to get an inert pill. After 42 days, the regimens were reversed. Participants and researchers didn't know which pill a volunteer was getting.

Every day throughout the study, each participant collected swabs of his or her genital area.

Although previous tests had shown that all the participants carried the genital herpes virus, some had never had an outbreak. Analysis of the swabs revealed that those asymptomatic people were as likely to shed the virus when they were getting the drug as when they received the placebo.

In contrast, famciclovir showed an effect in participants with histories of genital herpes outbreaks. This group was only about one-fourth as likely to shed virus while getting the drug as they were while getting the placebo, says Leone, who presented the findings last month at the 46th Interscience Conference on Antimicrobial Agents and Chemotherapy in San Francisco.

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