WASHINGTON (AP) -- The U.S. government approved a novel drug Monday 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. A spokeswoman said it would cost about the same as other HIV medicines, roughly $900 (euro651.3) a month wholesale.

It marks the first in a new class of HIV medicines since 2003, when the FDA approved an injectible "fusion inhibitor" that blocks a slightly later stage of infection.

Researchers have long known that people who naturally lack a working version of the CCR5 doorway are somewhat resistant to HIV infection, and slow to develop AIDS if they do become infected. But the race to develop CCR5 receptor blockers slowed with concerns that this family of drugs might cause serious side effects, including liver or heart damage, and an increased risk of other infections or cancer.

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. "That doesn't mean there aren't any risks."

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.

Those patients also must get a blood test to ensure the HIV strain they have uses the CCR5 pathway. Doctors would send a frozen blood sample to a San Francisco testing company, Monogram Biosciences.

Patients determined to have so-called "R5 virus" would take the twice-a-day pill in addition to their usual HIV drugs.

In a six-month study where patients added either Selzentry or a dummy pill to their regular medicine, Selzentry users were twice as likely to have their virus levels become almost unmeasurable, she said.

Topping the side-effect list is a stern warning that the drug may damage the liver, with symptoms that may at first mimic an allergic reaction. People who develop signs of hepatitis or an allergic reaction should see a doctor immediately.

Pfizer also warned that studies counted more cardiovascular problems, including heart attacks, in Selzentry users. Other side effects include dizziness, upper respiratory infections and fever.

Pfizer said it planned to use the brand name Celsentri outside the U.S. The FDA forced a spelling change for fear of mixups with similar-sounding drugs here, Birnkrant said.