This pill prevents HIV infection. So, why don’t more at-risk people take it?

Every morning Ryan Nistler, a 34-year-old Bostonian, swallows a little blue pill. The drug doesn’t alter his mental state. It doesn’t alleviate any illness. Yet he considers it essential to his well-being. Nistler’s morning pill is called Truvada, and it prevents him from getting HIV. Also known as PrEP — for pre-exposure prophylaxis — the medicine serves as a backstop to other methods of avoiding infection for those who are at high risk. It’s considered one of the most significant advances in the fight against AIDS. But less than one-tenth of those who could benefit from it take it.

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