The Food and Drug Administration has approved a new anti-AIDS pill, an option for hard-to-treat patients.

Manufacturer Merck & Co. said Isentress should be on pharmacy shelves within two weeks.

The AIDS virus uses three different enzymes to reproduce and infect cells. Numerous drugs are available that target two of those enzymes, called protease and reverse transcriptase.

Isentress is the first in a new class of medicines that blocks the third enzyme, called integrase. Added to "cocktails" of other HIV medicines, the drug can lower the amount of HIV in the blood and help infection-fighting immune cells rebound.

HIV mutates rapidly to resist various treatments, and the Food and Drug Administration approved use of Isentress in patients over age 16 whose blood tests show they are resistant to common older medications.

Side effects include diarrhea, nausea, headache and itching.

Patients take Isentress, also known as raltegravir, twice a day. A Merck spokeswoman said the drug would cost $27 a day, or $9,855 a year _ in the range of other competitors.

It is the second novel HIV drug to win FDA approval in two months. Pfizer Inc.'s Selzentry works by yet another method, blocking a passage that HIV often uses to enter white blood cells.

The Associated Press contributed to this story