A trial in Africa has failed to prove the effectiveness of using HIV medicine to prevent HIV infection in women, but it did show one thing: People, especially the young, are pretty reckless when it comes to sex.

The study, called Vaginal and Oral Interventions to Control the Epidemic, tested three ways of delivering a daily dose of antiretroviral drug to the body: via a tablet of Truvada, a tablet of tenofovir and a vaginal gel containing tenofovir. In previous trials, ARVs in pill and gel form have reduced HIV infections, so one might have expected the study volunteers  -- sexually active women in Uganda, South Africa and Zimbabwe, half of whom were unmarried -- to have taken them.

Yet the drugs failed to show efficacy, the trial partners believe, because so few of the women actually took them regularly. Among blood samples of women in the experiment group, the drug was detected only a third of the time among tablet-takers and a quarter of the time among those assigned to use gel.

The women knew what they were supposed to do. In interviews, they told trial workers that they were adhering to their daily regimens. And they turned in so few leftover pills and unused gel applicators that researchers had calculated, based on these measures, an adherence rate of 90 percent for the trial.

Though they weren't maximizing precautions, the women were taking risks. Of the 5,029 women enrolled in the study, 312 became HIV infected during its course, reflecting an incidence rate of 5.7 percent, more than twice what researchers had expected. Especially alarming was the incidence -- 8.8 percent -- for single women under 25, the group least likely to use their study product.

The study results suggest using a daily pill or gel dose as prevention is a lot like using a condom: effective for those who have the discipline to do it. For the rest, researchers continue to work on more forgiving methods, including ARV injections and vaginal rings as well as HIV vaccines.

(Lisa Beyer is a member of the Bloomberg View editorial board.)