San Diego ? For the first time, a drug widely used to treat genital herpes has been shown to prevent its spread as well, offering a new way of curbing an infection already carried by one in five Americans.

A study released Friday found that a once-daily Valtrex pill cut in half the risk of passing on the infection when taken by people with herpes simplex virus type 2, the primary cause of genital herpes.

Until now, the only advice for avoiding herpes during sex has been to use condoms. While no one knows which is more effective, condoms are clearly not foolproof, since the virus spreads by contact with herpes sores, and condoms may not cover them all.

Dr. Lawrence Corey, the study’s director, said the latest research suggests a new use for Valtrex in so-called discordant couples those in which one partner is infected and one is not. The drug is already widely prescribed to prevent and treat herpes flare-ups.

“If you ask infected people their biggest concern, it’s giving this to their loved ones,” he said.

Health experts said the study is especially noteworthy because it suggests that other, more serious sexually transmitted ills may also be controlled by treating the carrier.

“It’s a very important study, and it opens up the arena of treating discordant couples to prevent sexually transmitted disease,” said Dr. Scott Hammer of Columbia University. “This is a nuisance disease, but it lays the groundwork for other, life-threatening diseases, such as HIV.”

The Centers for Disease Control and Prevention estimates that 45 million American teenagers and adults are infected with the type 2 virus, which is almost always spread during sexual contact. The other herpes simplex virus, type 1, is much more common and causes cold sores. However, it too can cause genital infections if spread through oral-genital contact.

Usually, the type 2 virus causes only mild symptoms or no obvious sores at all. In fact, 90 percent of infected people do not realize they have it. Nevertheless, an unlucky minority suffers recurring painful genital sores.

In the latest study, doctors tested Valtrex known generically as valacyclovir on 1,484 couples in which one partner had recurring flare-ups with type 2 herpes and the other was not infected.

Corey, head of virology at the University of Washington in Seattle, presented the results at a meeting in San Diego of the American Society for Microbiology. The study was sponsored by GlaxoSmithKline, which makes Valtrex.

The volunteers were randomly given either daily Valtrex or dummy pills, offered advice on using condoms and then followed for eight months. Two percent of those taking Valtrex passed on the virus to their partners, compared with 4 percent on dummy pills.

The treatment nearly eliminated herpes symptoms in the partners, even if they caught the virus. Just half of 1 percent of those whose infected partners took Valtrex got herpes sores, compared with 2 percent in the comparison group.

“This is the first drug shown to interrupt the transmission of a sexually transmitted disease,” Corey said.

Doctors presume that AIDS drugs also slow transmission of HIV, although this has not been proven. A study intended to show this is in the works.

Valtrex is a modified version of acyclovir, the first herpes drug. Another herpes drug, Novartis’s Famvir, has not been tested for preventing transmission.