A drug used to treat HIV-positive patients may offer gay and bisexual men some protection against contracting the virus, the authors of a new study say.
Trials of the combination drug Truvada among nearly 2,500 men suggested it could reduce the chances of male-to-male HIV infection by 44%.
Those using the drug regularly could further reduce the risk of infection, it was claimed.
The study is published in the New England Journal of Medicine.
Pills and condoms
Truvada is the trade name of a drug manufactured by the California-based company Gilead Sciences Inc which combines two antiretroviral drugs, used to treat Aids.
But this new study looks at whether it could be used to prevent HIV infection in the first place.
Almost 2,500 gay or bisexual men were randomly selected in Peru, Ecuador, Brazil, South Africa, Thailand and the United States. Half were given the pill, half were given dummy tablets.
All the men were also given condoms and counselling on safe sex.
What the researchers found after about a year of testing was that the drug appeared to cut male-to-male HIV transmission by 44%, when the group taking the pill was compared with the placebo group.
Those who took the pill regularly were deemed to have reduced their risk of infection further, by up to 73%, and blood tests were run to confirm this relationship between pill-usage and protection levels.
The research was funded by the Bill & Melinda Gates Foundation, and the federal US body, the National Institute of Allergy and Infectious Diseases (NIAID). The pills were donated by their manufacturer.
NIAID director, Dr Anthony Fauci, conceded more work needed to be done, but called the results impressive.
"This has been done in men who have sex with men. We need to know if we get similar results in women as well as in heterosexual men, which we have reason to believe we will," he told the BBC.
"We also need to get a long term view of were there any toxicities. We didn't see anything that was significant but we need to follow that for a long period of time."
Questions and concerns
The trial does of course raise questions and concerns. Is it possible, for instance, that the results were skewed by greater condom use in the group that took the pill; and won't such findings encourage some men to dispense with condoms altogether in favour of a drug?
There is also the issue of prohibitive cost of Truvada, which retails in the US for around $36 a day, and which makes the drug unaffordable to many possible users.
Dr Fauci argues that the two groups were fully randomised and says that drugs can only play a complementary role in the war on HIV. Condoms and fewer partners, he said, remain the first line of defence.
"We're hoping that if this does become a useable tool in prevention, then the associated counselling will complement the effect of the drug and stop people becoming cavalier about it and say 'now I have a pill I don't have to worry'.
"That's exactly the opposite of what we want to happen. We want to add something rather than have it replace something."
Sir Nick Partridge, chief executive of the Terrence Higgins Trust, called the trial results "potentially significant".
"It's vital that we expand the ways we can prevent HIV transmission, particularly amongst those most at risk," he said in a statement. "This trial proves that HIV treatment will have an impact on prevention, but that it's not ready for widespread use yet.
"Three major hurdles are still going to be its cost, the risks of drug-resistant strains of HIV developing and taking a drug treatment every day."