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AIDS prevention got a boost last year when researchers reported that the daily combination anti-HIV pill Truvada, which is designed to treat the disease, may actually be able to prevent HIV infection among healthy, high-risk gay men.

Now the study's investigators have halted another arm of the trial — this one involving women — after early results showed that Truvada did nothing to lower their risk of infection with HIV. The pill, which is a combination of two anti-HIV drugs, tenofovir and emtricitabine, was pitted against a placebo and worked no better. (More on TIME.com: Senior Class Project: HIV Tests for All. Is This Really a Good Idea?)

The results appear contrary to the initial positive data, which we reported here, but, says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, it's still too early to draw any conclusions about the study in women. The trial focused on commercial sex workers, who are at increased risk of acquiring HIV, and, says Fauci, may not be reporting their adherence to the drug regimen accurately. There was evidence of high rates of pregnancy among the participants who were also taking birth control pills, which suggests there may be some discrepancy between the reported and actual use of the anti-HIV drugs.

Aside from reporting issues, there could also be a biological explanation of why the protective effect found among gay men was not seen in women. There may be a physiological difference in how men and women acquire HIV, since different tissues are involved during sexual contact — vaginal mucosa for women, and rectal mucosa for men. HIV may preferentially infect one type of tissue over the other, or there could be other physiological reasons for the gender-based differences in infection rates. (More on Time.com: An Anti-HIV Gel Shows Some Promise for Prevention)

Alternatively, it may be that different tissues retain different concentrations of HIV-fighting drugs, which could help explain why the medications may work in blocking infection among men but not in women.

"The results are disappointing, and a little surprising, but not totally surprising," says Fauci. "One or the other or both of these types of factors — whether it's behavioral and has to do with adherence to the drug regimen, or whether it's the concentration of the drugs in the tissues — is almost certainly going to have a part in the explanation of these results." (More on TIME.com: Perspective: To Prevent AIDS in Russia, Drug Addicts Need Care)

What impact do the findings have on the prospect of using anti-HIV treatments to prevent, rather than just treat, HIV infection? "There are still too many unanswered questions for us to make any conclusions about the role of (using anti-HIV medications to prevent HIV) in women," he says. But given the promising data among gay men, Fauci says further study among women may be justified, in order to figure out whether there really is a fundamental difference between men and women that prevents women from protection with Truvada, or whether a shortcoming of the current study — in failing to document women's adherence to the medication — is to blame.

• Alice Park