The approval marks the first time an HIV drug has been approved for people who have not yet been infected. It is approved for those at high risk of infection and those who may engage in sexual activity with HIV-infected partners.

The drug, taken daily, is to be used in combination with safer sex practices to reduce the risk of sexually acquired HIV infection in adults at high risk.

Truvada has already been approved in combination with other agents as a treatment for HIV-infected adults and children older than 12.

"Today's approval marks an important milestone in our fight against HIV," said Margaret Hamburg, Commissioner of the U.S. Food and Drug Administration, in a statement.

Every year, about 50,000 U.S. adults and adolescents are diagnosed with HIV infection, Hamburg said. "New treatments as well as prevention methods are needed to fight the HIV epidemic in this country."

As part of the approval, the FDA is strengthening the warning label on the drug to make sure physicians screen individuals to ensure they are not infected with the HIV virus before prescribing the medication and at least every three months during use.

The approval includes a risk mitigation program that includes training and education to assist prescribers in counseling people considering Truvada as a prophylaxis.

As a condition for approval, Gilead is required to collect samples from individuals who acquire HIV while taking Truvada and to evaluate them for potential resistance to the drug. It is also required to collect data on pregnancy outcomes for women who become pregnant while taking Truvada as prophylactic.

Gilead's shares rose 1.1 percent to $51.74 in afternoon trading on the Nasdaq.