Researchers emboldened by monkey study results recently said they would expand tests of the pill Truvada, which combines Gilead Science Inc.'s Viread and Emtriva, as a possible preventive for healthy people at high risk of HIV infection. Gilead, however, has acted to tamp down excitement about the development, in part out of fear that Truvada will be seen as a "bio-medical condom." Health officials, too, are worried: They say the drugs should only be administered along with counseling, condoms and regular testing.

Truvada, Viread and Emtriva are approved for use by HIV-positive patients, not as a preventive for HIV-negative people. Doctors, however, can prescribe them for "off-label" use; in addition, they are available online. No one knows how many people are taking the pills for prevention, but some experts, including Mitchell Warren of the AIDS Vaccine Advocacy Coalition, suspect their numbers are growing. Some analysts say this off-label use could come back to haunt Gilead, in particular if the pills are eventually found to be ineffective at preventing infection.

CDC, the National Institutes of Health and the Bill & Melinda Gates Foundation are each funding separate human trials of Viread around the world to learn whether it could be an effective prevention pill. Other than providing the Viread, Gilead is not deeply involved in these studies. The idea that any of their drugs can be used to prevent AIDS is promising but preliminary, company officials say, because most of the available data come from monkey studies. But because the monkey trial results have been so promising, officials are now planning to switch one human trial from Viread to the combination.

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