As expected after a unanimous advisory panel vote last month, the FDA approved Merck's (NYSE:MRK) HIV drug ISENTRESS on Friday. While that's good news for HIV patients, Merck's investors will have to wait a while before they can truly reap the financial benefits of this potential blockbuster drug.

The drug was approved under an accelerated FDA approval process that allows drugs that treat diseases with unmet needs to be approved with less safety and efficacy data than is normally required. But that speed comes with a price -- ISENTRESS is only approved for use in patients who have already failed other safety-proven treatments.

The drug is the first in a class of integrase inhibitors, which block the viral DNA from integrating with the patient's DNA. Because it blocks a new mechanism of viral replication, the drug works well in patients who have developed resistance to current treatments, which block viral replication by attacking the protease or reverse transcriptase enzymes.

The drug should also work in patients who haven't gained resistance to other drugs, but Merck will need to complete ongoing phase 3 clinical trials in patients who aren't resistant before it can gain marketing approval for that segment of the patient population.

Merck should have the integrase inhibitor market to itself for a while. Its closest competitor is Gilead Sciences (NASDAQ:GILD), which completed a phase 2 clinical trial earlier this year. Merck's big pharma brethren GlaxoSmithKline (NYSE:GSK) and Bristol-Myers Squibb (NYSE:BMY) also have integrase inhibitors, but they're further behind in the clinic.

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