WASHINGTON (Reuters) - The U.S. Food and Drug Administration on Friday approved GlaxoSmithKline's drug Valtrex (valacyclovir) for use in preventing transmission of genital herpes.

Valtrex, a successor to acyclovir, was already approved for treating and suppressing genital herpes. The new approval allows it to be marketed for preventing transmission.

"Valtrex reduces the risk of heterosexual transmission of genital herpes to susceptible partners with healthy immune systems when used as suppressive therapy in combination with safer sex practices," the FDA said in a statement.

Herpes is one of the most common sexually transmitted diseases. Hundreds of millions of people around the world are infected, the World Health Organization says, with up to 70% of the population in some African countries infected and 22% in the United States.

The FDA advises herpes-infected patients to always use a condom when having sex to reduce the risk of infecting someone else. It also advises avoiding sex during an outbreak.

The FDA said a study of 1,500 monogamous, heterosexual couples showed that, over 8 months, patients who took Valtrex were 48% less likely to pass the virus to their partners.

It has potential side effects, including kidney problems and nervous system problems, such as aggressive behavior, unsteady movements, shaky movements, confusion, speech problems, hallucinations, seizures and coma.

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