(Reuters) - Health regulators for the first time approved using an HIV drug to prevent infection in people who are at high risk of contracting the virus that causes AIDS.

Gilead Sciences Inc's Truvada was approved for people who are not yet infected but may engage in sexual activity with HIV-infected partners, a concept known as pre-exposure prophylaxis (PrEP).

Truvada, which combines two anti-HIV drugs in one pill, is already approved in combination with other antiretroviral agents to treat patients 12 years of age and older who are infected with the virus.

Under the new Food and Drug Administration approval Truvada taken daily, is to be used along with safer sex practices, risk reduction counseling and regular HIV testing to reduce the risk of sexually acquired HIV infection in adults deemed to be at high risk.

Today's approval marks an important milestone in our fight against HIV, FDA Commissioner Margaret Hamburg said 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.

A landmark study published in 2010 found that giving a daily dose of Truvada to men who have sex with other men could reduce HIV infection rates by 44 percent.

The use of combination therapies for people already infected with HIV has turned the virus that was once a sure death sentence into a condition people can live with for many years.

Meanwhile, researchers have been working for decades to come up with a vaccine that would prevent the virus from infecting people only to be disappointed time and again. A 2009 clinical trial in Thailand was the first to show it was possible to prevent HIV infection in humans, and more powerful vaccines using HIV-fighting antibodies are being studied amid renewed optimism that a vaccine may be within reach.

Debra Birnkrant, director of the Division of Antiviral Products at the FDA, said the federal government has a goal of a 25 percent decrease in HIV infections in the United States by 2015, and added that responsible use of Truvada for prevention could help achieve it.

In a conference call with reporters, Birnkrant was asked if the drug might give people a false sense of security that could tempt them to stop using condoms or otherwise engage in unsafe sex.

We are concerned about risk-taking behavior, however, the (clinical) trials did not bear that out, she said. In one U.S. trial involving 400 men who have sex with other men, she said there was not an increase in unsafe sex.

The hope is the right messages will get out and Truvada will be used properly, she said.

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 as a preventive measure, and at least every three months during use.

The approval comes with 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 closed up 1.5 percent at $51.94 on Nasdaq.

(Reporting by Toni Clarke in Boston and Ransdell Pierson and Bill Berkrot in New York; Editing by Maureen Bavdek, Bernard Orr)