Researchers have released the final results of two studies that suggest AIDS drugs can prevent exposed people in Africa from getting infected with HIV by their sexual partners. However, another study indicates that it’s a tough job to convince African women who aren’t at the highest risk to take preventive medications.In the big picture, the studies show that "we have a new HIV-prevention strategy, one that’s quite powerful but also depends on adherence,” said Dr. Jared Baeten, an associate professor of global health at the University of Washington, in Seattle. "The next step is figuring out how to motivate people to take it.” The studies appeared online July 11 in the New England Journal of Medicine.The general findings of the studies have been previously released, but only now has the research become available in a medical journal after going through a peer-review process.Two studies offer promising details about the potential for the drugs to prevent -- although not all the time -- the transmission of HIV to heterosexual men and women from their infected partners. One study in Kenya and Uganda looked at heterosexual couples -- almost all married -- in which one person was infected with HIV, the virus that causes AIDS. The uninfected partners were randomly assigned to take an inactive placebo or a once-daily dose of the drug tenofovir (Viread) or a tenofovir-emtricitabine combination (Truvada) for up to three years. Nearly 5,000 people completed the study. Those who took Truvada had a 75 percent lower risk of becoming infected with HIV compared to those who received a placebo. The risk was 67 percent lower in those who took Viread compared to a placebo. Even in those who got the placebo, the overall risk of getting infected was low: 52 of 1,468, or a little more than 3 percent, did so.Truvada treatment in the United States costs several thousand dollars a year, Baeten said, but the discounted price can be as cheap as 25 cents a day in Africa. The drug, which stops the AIDS virus from reproducing in people who are infected, appears to do the same thing in uninfected people who are exposed to the virus, he said. In their cases, the virus doesn’t already have a foothold in the body so it dies off.In this study, 10 percent or less of those who took the drugs reported side effects such as fatigue, diarrhea and nausea, and only in the first month. The second study of 1,219 HIV-negative adults in Botswana looked at Truvada versus a placebo. Comparing the 33 participants who became infected during the trial -- nine people in the drug group and 24 people on a placebo -- the study found those who took Truvada were 62 percent less likely to become infected with HIV.In this study, significant loss of bone mineral density was a side effect for participants receiving the drug, compared to those on a placebo.Another study, in Kenya, South Africa and Tanzania, assigned 2,120 women at higher risk of HIV infection to receive Truvada or a placebo. However, there wasn’t much difference in HIV infection rates between the two groups -- about 5 percent in both became infected. Baeten explained the finding, saying many women stopped taking the drug, which prevented an accurate assessment of its effectiveness. The next step in research into the use of the drugs to prevent infection is to "figure out how to make them work in the real world, outside of an intensive research setting,” Baeten said. In the United States, for example, researchers are studying their use in gay men who are at high risk for infection. As for condoms, another major player in HIV prevention, Baeten said the prevention drugs will add to their level of security or provide some protection in cases where people can’t use condoms.In an editorial accompanying the studies, two experts stressed that medications should never be viewed as a substitute for the condom."Although no evidence of increased risky sexual behavior or decreased condom usage was reported in these studies, we must ensure that pre-exposure prophylaxis does not indirectly encourage such behavior,” wrote Dr. Myron Cohen of the University of North Carolina at Chapel Hill and Dr. Lindsey Baden of Brigham and Women’s Hospital, Boston. They added that more research is needed to properly assess who stands to benefit most from these drug regimens, the best timing and dosage, as well as any potential side effects from long-term use.