An HIV prevention pill taken by gay men before and after sex reduced the risk of virus transmission by 86 percent, according to the results of a clinical trial released Tuesday.

Truvada contains HIV treatment drugs tenofovir and emtricitabine, and is made by Gilead Sciences in California. In the study Truvada was taken up to one day before sex and the two days after.

The study, led by the French National HIV research agency ANRS, was presented at the Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle. Until now, Truvada as a preventive measure has been prescribed only as a once-a-day regimen, not one that could be effective if taken around the time of sex.

While the latest evidence on the technique known as pre-exposure prophylaxis, or PrEP, is not enough to change guidelines, it was welcomed as a novel approach to HIV prevention.

The study "provides the first evidence that an event-driven regimen is effective among high-risk MSM (men who have sex with men) with frequent sex," said Jonathan Mermin, director of the US Centers for Disease Control and Prevention's National Center for HIV/AIDS.

Truvada was approved in 2012 by the US Food and Drug Administration as the first daily pill to help prevent human immunodeficiency virus in some high-risk groups.

Three pills

The latest trial, called IPERGAY, was a randomized, placebo-controlled study that began in February 2012 and was halted in October 2014 when an independent data safety review board found it had a high rate of effectiveness. A total of 400 participants were enrolled.

Those in the trial were considered to be at high-risk of acquiring HIV because they were having sex an average of 10 times per month, and with eight partners every two months.

Some participants took a placebo, while others took Truvada two to 24 hours before sex, then again 24 and 48 hours later.

The research team "reported that PrEP reduced HIV risk among the MSM who were prescribed this regimen by an average of 86 percent," according to a statement.

Previous research on Truvada to prevent HIV has found that those who took it regularly could lower their risk of getting HIV by as much as 92 percent.

Most of the men in the study were taking Truvada four days a week, and it remains to be seen if the regimen would be as effective among men having sex less frequently.

Mermin cautioned that it remains unknown whether Truvada taken only in the hours before sex and the two days afterward could work in heterosexual men and women, or injection drug users, or outside a tightly controlled trial."CDC continues to recommend only daily use of PrEP, as approved by the FDA," Mermin said."

Daily, oral PrEP is recommended by CDC as an option for men who have sex with men, heterosexual men and women, and injection drug users."

More PrEP study

A second study released at CROI by the UK Medical Research Council found that daily oral PrEP using Truvada reduced risk of HIV transmission among gay men by 86 percent.

A third study examined the use of a daily pill to prevent HIV among heterosexual couples in which one person is HIV positive and the other is not.

Led by the University of Washington, the trial underway in Kenya and Uganda is probing the efficacy of PrEP as a transitional strategy, to protect one partner from HIV while the HIV-positive partner gets his or her viral load under control through antiretroviral medication."

Preliminary results show that this dual strategy reduced the risk of HIV infection by 96 percent," Mermin said."

"These findings together provide encouraging additional evidence of the power of PrEP to reduce the risk of HIV infection."