In a statement Merck said more testing was necessary before it was approved for new HIV patients or children.
Isentress belongs to a class of drugs called integrase inhibitors. These drugs work by blocking the integrase enzyme, which helps HIV replicate by inserting its DNA into new cells. Isentress is the first drug in the class to win FDA approval.
Dr. Anthony Fauci, a noted HIV expert and director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, said the FDA's approval "will be most welcome in the community of physicians taking care of HIV-infected patients."
"Its mechanism of action is particularly important in that it blocks the ability of the virus to integrate itself into the genes of cells," Fauci said. "This property of the virus to integrate is important in establishing the reservoir of virus in the body that has made it extremely difficult to eradicate HIV, even with prolonged treatment."
Two earlier classes of anti-HIV drugs -- protease and reverse transcriptase inhibitors -- also work by blocking different enzymes involved in HIV replication.
Friday's decision by the FDA will give doctors a new tool to help patients who have developed resistance to existing drugs or who are infected with drug-resistant strains of HIV.
Like protease and reverse transcriptase inhibitors, Isentress will also be prescribed for patients in combination with other drugs to maximize the number of ways the virus is being attacked.
The cost of the recommended daily regimen of Isentress -- a 400 mg tablet taken twice a day -- will be comparable to protease inhibitors, with a wholesale price of $27, Merck said.