A pill used to treat people with HIV when given to healthy gay and bisexual men during a three-year global trial lessened their chances of infection, says a new study.
The drug Truvada lowered the chances of infection by 44 percent, and by 73 percent or more among men who took their pills most faithfully.
The study involved about 2,500 men at high risk of HIV infection in Peru, Ecuador, Brazil, South Africa, Thailand and the United States.
Researchers had feared the pills might give a false sense of security and make men less likely to use condoms or to limit their partners, but the opposite happened - risky sex declined.
The results are `a major advance` that can help curb the epidemic in gay men, said Kevin Fenton, AIDS prevention chief at the US Centers for Disease Control and Prevention (CDC), reports the Daily Mail.
But he warned they may not apply to people exposed to HIV through male-female sex, drug use or other ways. Studies in those groups are under way, according to a CDC statement.
Because Truvada is already in the market, the CDC is rushing to develop guidelines for doctors who want to use it to prevent HIV, and urged people to wait until those are ready.
However, high prices of the pills which cost $5,000 to $14,000 a year in the US could be prohibitive.
Whether insurers or government health programmes should pay for them is one of the tough issues to be sorted out, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
"This is an exciting finding" but it "is only one study in one specific study population", so its impact on others is unknown, Fauci said.