The first-ever daily pill to help prevent the spread of HIV has been approved by US regulators - a milestone in the 30-year battle against the virus believed to cause AIDS.
The US Food and Drug Administration on Monday approved Gilead Sciences' pill Truvada as a preventive measure for people who are at high risk of acquiring HIV through sexual activity, such as those who have HIV-infected partners.
Public health advocates said the approval could help slow the spread of HIV, which has held steady at about 50,000 new infections per year for the last 15 years in the US.
An estimated 1.2m Americans have HIV, which scientists believe develops into AIDS unless treated with antiviral drugs.
With 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.
Gilead Sciences has marketed Truvada since 2004 as a treatment for people who are already infected with the virus.
But starting in 2010, studies showed that the drug could actually 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 per cent, when accompanied by condoms and counselling.
Last year another study found that Truvada reduced infection by 75 per cent in heterosexual couples in which one partner was infected with HIV and the other was not.
Because Truvada is on the market to manage HIV, some doctors already prescribe it as a preventive measure.
FDA approval will allow Gilead Sciences to formally market the drug for that use, which could dramatically increase prescribing.
However, the regimen is estimated to cost around $14,000 per year, putting it out of reach for many.
Common side effects of the pill include diarrhoea, nausea, abdominal pain, headaches and weight loss.
Truvada's ground-breaking preventive ability has also exposed disagreements about managing the disease among those in the HIV community.
Groups including the AIDS Healthcare Foundation asked the FDA to reject the new move, saying it could give patients a false sense of security and reduce the use of condoms, regarded as the most reliable preventive measure against HIV.
But FDA scientists on Monday said there was no indication from clinical trials that Truvada users were more likely to engage in risky sexual behaviour.
"What we found was that condom use increased over time and sexually transmitted infections either remained at baseline levels or decreased," said Dr Debra Birnkrant, FDA's director of antiviral products.
"So in essence, we don't have any strong evidence that condoms were not used or there was a decrease in condom use."
The goal of the approval is to eventually cut back on the rate of new infections in the US, which have stayed steady in recent years, she said.
A key goal of the US strategy against HIV/AIDS, set forth in 2010, is to decrease the number of new infections by 25 per cent by 2015.
"The hope is that over time it will decrease the rate of new infections or incidence in the United States."