With donated drugs and services provided by major pharmacy chains, 200,000 uninsured Americans will gain access to H.I.V.-preventive medicines at no cost, the Trump administration announced on Tuesday.

The announcement, by Alex M. Azar II, the health and human services secretary, essentially explained how the government plans to distribute the drugs for pre-exposure prophylaxis, or PrEP, that were promised in May by the drugmaker Gilead Sciences.

PrEP describes a strategy of preventing infection with H.I.V. by taking a single pill a day, either Truvada or Descovy. Both are made by Gilead. The strategy is 99 percent effective at preventing infection, studies have shown, and is a mainstay of the administration’s campaign to end the H.I.V. epidemic.

Some American cities with high H.I.V. rates, such as San Francisco, already have programs that pay the costs of PrEP for the uninsured. Gilead itself offers the drug at no cost to those who cannot afford it, or picks up insurance co-pays for patients who qualify.