The drug can significantly reduce new HIV infections when taken daily and used correctly. One big aspect of correct usage? Condoms:

If approved, the drug would be recommended for H.I.V.-negative people at high risk of becoming infected, like gay men who have multiple partners and do not use condoms consistently, prostitutes and people in relationships with someone who is H.I.V.-positive.... Perhaps the most serious concern is that some who take the drug will assume they no longer need condoms. But condoms are still necessary, because Truvada is not 100 percent effective. If people take the drug inconsistently and also skip condoms, they may wind up at even greater risk than they were before Truvada came along.

So guys who don't care enough about their health or the health of their partners to consistently use condoms in the first place will magically start using condoms once they're taking this drug. And this group of men—guys who don't think clearly about health issues and who also have trouble assessing risks or anticipating negative consequences—will need to take this drug daily, they'll have to take it religiously, otherwise...

Then there is the question of adherence. Skipping doses or using Truvada on occasion, as if it were a party drug, would invite both infection and the emergence of drug-resistant strains. But research shows that people are not good at sticking to the required daily regimen. In one major study, only 10 percent of the participants took Truvada as directed.

How does this drug help exactly? How does it not make things worse?