The Food and Drug Administration has approved the first drug shown to reduce the risk of HIV infection, a milestone in the 30-year battle against the virus that causes AIDS.

The agency approved Gilead Sciences' pill Truvada as a preventive measure for people who are at high risk of acquiring HIV, such as those who have sex with HIV-infected partners.

Gilead Sciences Inc. has marketed Truvada since 2004 as a treatment for people who are infected with the virus.

But company studies have shown the drug can prevent people from contracting HIV when used as a precautionary measure. A three-year study found that daily doses cut the risk of infection in healthy gay and bisexual men by 42 percent, when accompanied by condoms and counseling.