The Food and Drug Administration on Thursday approved a new drug, Descovy, for prevention of infection with H.I.V., only the second drug approved for this purpose.

The first, Truvada, has become a mainstay of government efforts to turn back the H.I.V. epidemic. But the F.D.A.’s approval of Descovy explicitly excludes “individuals who have receptive vaginal sex,” which may include cisgender women and many transgender men, and does not outline a plan for making the drug available to them.

The drug’s maker, Gilead Sciences, tested it only “in men and transgender women,” the F.D.A. noted. Some activists and scientists said the approval sets a dangerous precedent by allowing companies to dodge the expensive trials needed to test medicines in cisgender women and other groups at risk of H.I.V. infection.

Such an exclusion “should be unacceptable in these days and times,” said Dr. Rochelle Walensky, chief of infectious diseases at Massachusetts General Hospital.