TUESDAY, Nov. 23 (HealthDay News) -- A pill a day cut the risk of HIV infection by almost 44 percent in those at highest risk for contracting the virus, namely sexually active gay and bisexual men, a new study finds.

The reduction in risk climbed to nearly 73 percent among study participants who took the pill, called Truvada, 90 percent of the time, the researchers added. Truvada is already available by prescription in the United States as a treatment for HIV-infected people.

The pill, which was tested in over 2,500 men at 11 sites in six countries worldwide, combines two HIV antiretroviral medications, emtricitabine (Emtriva) and tenofovir (Viread).

Experts are hailing the results as a big step in the worldwide effort to combat the spread of the HIV virus, which infected some 2.7 million people worldwide in 2008.

And it adds to other efforts to prevent spread, such as a topical gel containing tenofovir that was found last summer to be effective in preventing HIV infection in women in southern Africa.

"These results represent a major advance in HIV prevention research," Dr. Kevin Fenton, director of the National Center for HIV/AIDS, Viral Hepatitis, STD & TB Prevention at the U.S. Centers for Disease Control and Prevention, said in a statement. "For the first time, we have evidence that a daily pill used to treat HIV is partially effective for preventing HIV among gay and bisexual men at high risk for infection, when combined with other prevention strategies. Given the heavy burden of HIV among gay and bisexual men, a new tool with potential additive benefit is exciting and welcome news."

Still, said an expert, this is hardly the last battle in the war against HIV/AIDS.

"This really is a significant study that showed promising results, but there are issues because of the high adherence required," said Dr. Michael Horberg, director of HIV/AIDS at Kaiser Permanente in Santa Clara, Calif. "There are issues of its long-term use and further trials are needed, as indicated by the authors."

Previous studies had found that antiretroviral drugs given to animals before exposure to the AIDS virus did protect them against infection.

And antiretrovirals are also used to prevent infection in people who have been exposed to HIV-infected fluid; those regimens need to start within 72 hours of exposure.

The current study, published in the Nov. 25 issue of the New England Journal of Medicine, is the first human trial to look at the effectiveness of this combination in prevention.

For this large study, about 2,500 men or transgender women (men who consider themselves women but are born male) who have sex with men were randomly assigned to receive Truvada or a placebo once daily.

The pills were provided by Gilead Sciences, which makes Truvada, although the study was funded by the U.S. National Institutes of Health and the Gates Foundation.

Both groups also received intensive counseling on safe sex practices and were given condoms (about 500,000 condoms were distributed throughout the course of the trial).

The researchers explain that the trial participants were assumed to be at extremely high risk for HIV infection, having had an average of 18 sexual partners in the past 12 weeks.

In all, 64 people in the placebo group of 1,248 were infected with HIV over the course of the study (average follow-up was 1.2 years) as opposed to 36 among the 1,251 participants receiving the drug.

Although the average risk reduction in the entire study population was 44 percent, it went as high as 72.8 percent among those who took the pill at least 90 percent of the time.

The authors were able to verify compliance by measuring blood levels of the drugs, and levels of the drug in participant's blood seemed to correlate with HIV status 51 percent of HIV-negative people showed detectable levels of the drug in blood samples versus just 9 percent of those who became HIV-positive, the researchers reported.

By contrast, about 3 percent of a similar group of men who received neither the drug nor condoms or counseling would be expected to contract HIV over the same period.

"If you did nothing, you would expect at least 3 percent of men to get infected, particularly if you're talking about young men," said one of the study's principal investigators, Dr. Kenneth Mayer, medical research director and co-chair of The Fenway Institute at Fenway Health in Boston, one of the sites participating in the study.

Side effects included nausea, headaches, weight loss and elevated levels of creatinine (a naturally occurring molecule filtered by the kidneys). The researchers say that while adverse events were similar between Truvada users and non-users, more people who took the pill suffered from nausea.

Mayer believes that adherence might actually go higher in the real world, now that the efficacy of the drug combo has been shown.

"There was no human data before that said taking pills in advance would protect you. Now we have the data," he said.

"Protection was over 90 percent when the drug was in the body," Mayer said. "That says to us that we can probably do better in adherence. People may be more motivated to adhere once they [see these results]."

But translation into the real-world would take a "massive commitment" of federal dollars along with concerted efforts on the parts of communities to reach out to a vulnerable population which often mistrusts the health-care system, Horberg said.

There's the cost for the medication, for one thing: A year's worth of Truvada totals more than $10,000 a year, and that is the discounted rate, Mayer said. Although generic versions are available in some countries (not the United States) for as low as $500 a year, he added.

Researchers are now looking at whether the drug might also be effective in other populations, including women.

The CDC issued a statement Tuesday hailing the study findings and saying it would "fully review the trial data and publish interim guidance for physicians" in the coming weeks.

But in the meantime, the agency said that use of the pill "should never be seen as the first line of defense against HIV," noting that the strategy was only partially effective, even when used with standard safe-sex approaches such as HIV testing and condom use.

More information

There's more on HIV/AIDS at the World Health Organization.