WASHINGTON—The U.S. government approved a novel drug Aug. 6 to help patients with the AIDS virus who are running out of options, while acknowledging lingering questions about the pills’ long-term effects.

Pfizer Inc.’s Selzentry is the first anti-AIDS drug that works by blocking a crucial doorway called the CCR5 receptor that the HIV virus often uses to enter white blood cells.

New York-based Pfizer said the pill, known chemically as maraviroc, would be available next month.

The Food and Drug Administration approved Selzentry after concluding that certain hard-to-treat patients need the new option, but is requiring Pfizer to conduct further research to assess long-term side effects, said Dr. Debra Birnkrant, the agency’s HIV drugs chief.

For that select group, Selzentry’s “benefits clearly outweigh the risks,” Birnkrant said.

The drug is not for the newly diagnosed, but only for patients whose virus is fast becoming resistant to today’s HIV drugs, she stressed.