The first drug shown to help prevent HIV infection has been endorsed by a panel of US expert, paving the way for the likely approval of its use by those at high risk.
In a series of votes, a US Food and Drug Administration advisory panel recommended the daily pill Truvada be allowed to be marketed as a preventative measure against the virus that causes AIDS.
Truvada first made headlines in 2010, when government researchers showed it could prevent people from contracting HIV.
A three-year study found that daily doses cut the risk of infection in healthy gay and bisexual men by 42%, when accompanied by condoms and counselling.
Last year another study found that Truvada reduced infection by 75% in heterosexual couples in which one partner was infected with HIV and the other was not.
However, the expert panel highlighted a number of concerns about the use of Truvada, including whether its use might lead to reduced use of condoms, the most reliable defense against HIV.
They also questioned the drug's effectiveness in women, who have shown much lower rates of protection in studies.
Doctors, nurses and patients have, meanwhile, expressed their concerns that people will not take the drug as recommended - every day, in addition to using condoms.
"Truvada needs to be taken every day, 100% of the time, and my experience as a registered nurse tells me that won't happen," Karen Haughey told the panel.
"In my eight years, not one patient that I've cared for has been 100% adherent."
An estimated 1.2 million Americans have HIV, which develops into AIDS unless treated with antiviral drugs.
The number of new HIV infections in the U.S. has held steady for 15 years at about 50,000 per year but with no vaccine in sight and an estimated 240,000 HIV carriers unaware of their status, doctors and patients say new methods are needed to fight the spread of the virus.
Truvada is already marketed as a treatment for people who are infected with the virus.
It is a combination of two older HIV drugs, Emtriva and Viread. Doctors usually prescribe it as part of a drug cocktail to repress the virus.
The panel's recommendation will now be forwarded to FDA regulators, who must decide whether to give Truvada final approval. Normally they follow the panel's advice.