Gilead Sciences' Truvada was approved for people who are not infected but might have sex with HIV-infected partners.

Truvada, which combines two anti-HIV drugs in one pill, is already approved in combination with other antiretroviral agents to treat HIV patients older than 11.

Under the new Food and Drug Administration approval, Truvada, taken daily, is to be used along with safer sex practices, risk reduction counselling 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.

Every year, about 50000 US adults and adolescents are diagnosed with HIV, Hamburg said.

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%.

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.

A 2009 clinical trial in Thailand was the first to show that it was possible to prevent HIV infection in humans. More powerful vaccines, using HIV-fighting antibodies, are being studied amid optimism that a vaccine might be within reach.

Debra Birnkrant, director of the division of antiviral products at the FDA, said the US government has the goal of a 25% decrease in HIV infections in the US by 2015, and added that responsible use of Truvada for preventing infection could help achieve it.

Birnkrant was asked if the drug might give people a false sense of security, prompting them to engage in unsafe sex.

"We are concerned about risk-taking behaviour but the [clinical] trials did not bear that out," she said.

In one US trial, involving 400 men who have sex with other men, she said there was not an increase in unsafe sex.

As part of the approval, the FDA is strengthening the warning label on the drug to make sure that doctors screen people to ensure that they are not infected with the HI virus before prescribing the medication as a prophylactic.

The approval is contingent on the implementation of a risk-mitigation programme that includes training and education to assist prescribers in counselling people considering using Truvada as a prophylactic.