The FDA has given its seal of approval to a drug that can cut the risk of HIV infection by three quarters.
Truvada - emtricitabine/tenofovir disoproxil fumarate - is already used as a treatment for HIV, but has now been shown to have a preventative effect too.
It can be used, together with safer sex, to lower peoples' chances of becoming infected with HIV when exposed to the virus. It's not a silver bullet, and will only be given alongside a training and education program.
"Today’s approval marks an important milestone in our fight against HIV," says FDA commissioner Dr Margaret A Hamburg.
"Every year, about 50,000 US adults and adolescents are diagnosed with HIV infection, despite the availability of prevention methods and strategies to educate, test, and care for people living with the disease. New treatments as well as prevention methods are needed to fight the HIV epidemic in this country."
Truvada was tested on 2,499 HIV-negative men or transgender women who have sex with men, all of whom showed evidence of high risk behavior for HIV infection: inconsistent or no condom use during sex with a partner who might be infected, for example, a high number of sex partners or prostitution.
The results showed Truvada cut the risk of HIV infection by 42 percent.
And it was even more effective in heterosexual couples where one partner was HIV-infected and the other was not, reducing the risk of infection by 75 percent.
The FDA's ordered Truvada’s manufacturer, Gilead Sciences, to collect viral isolates from anyone who acquires HIV while taking Truvada and to check them for resistance to the drug. It's also required to examine the effects on any women that become pregnant.
There is one obvious danger with the drug - quite apart from its $13,900-per-year price tag. There's a danger that taking it could make people blase about taking safe-sex precautions.
But it's the nearest thing yet to a vaccine against HIV, which currently affects as many as 1.2 million Americans, many of whom have partners at risk of the disease.
"This approval is a major milestone in our 30-year fight against AIDS," says Robert M Grant of the Gladstone Institute of Virology and Immunology, lead investigator of the clinical trial.
"The use of [pre-exposure prophylaxis] alongside routine HIV testing gives us a tremendous opportunity to reduce the rate of new HIV infections in this country and around the world."