In the next few months, said Mitchell Warren, executive director of AVAC, an advocacy group for AIDS prevention, “we’re going to see a cascade of results” from trials of what is called “oral pre-exposure prophylaxis,” or “oral prep” for short. In them, men and women who are not infected with the AIDS virus but who regularly engage in high-risk sex, like anal sex without condoms or sex for money with strangers, take a daily dose of one or two of the antiretroviral drugs normally taken by infected people.