The Food and Drug Administration usually takes about a month to make a decision about how long a drug will have exclusivity. Unless you're Amarin (NASDAQ:AMRN). Then it takes about 19 months.

And the answer isn't exactly what investors were hoping for.

On Friday, Amarin said the FDA had given its fish oil drug Vascepa three years of marketing exclusivity. The biotech was hoping the agency would designate Vascepa as a New Chemical Entity, entitling it to five years of exclusivity.

Vascepa

Source: Amarin.

Vascepa is a purified fish oil, which competes with GlaxoSmithKline's (NYSE:GSK) fish oil Lovaza. After much deliberation, the agency apparently decided that Vascepa wasn't different enough from GlaxoSmithKline's drug to justify designating it as a New Chemical Entity.

The New Product designation only comes with a three-year exclusivity period before generic drug companies can launch a generic version. Unfortunately, the exclusivity period started back when the drug was approved in July 2012, so there's only just more than a year left on the exclusivity period.

Amarin can buy itself a little more time by suing any companies that ask the FDA to approve generic versions of Vascepa; the agency holds the application for 30 months to let the patent validity be decided in court. Amarin has patents on Vascepa that extend to 2030, so if they're validated by the court, the difference between three and five years of exclusivity, which run concurrently to the patents, is essentially meaningless.

And all of this assumes a generic-drug maker would want to sell a generic version of Vascepa. Generic drugs are a game of volumes, and there were only about 75,000 prescriptions for Vascepa in the third quarter of 2013. Amarin will release fourth quarter results later this week, but I don't expect prescriptions to be substantially higher.

To really accelerate growth, Amarin needs the FDA to approve Vascepa in patients with moderately high triglyceride levels. The company is currently appealing the FDA's decision to rescind their agreement that the agency would approve the expanded indication if Amarin had an outcomes trial substantially enrolled. It appears the FDA now wants to see the results of that trial to confirm the drug is really preventing heart attacks and strokes before approving the expanded indication.

Results from the trial, dubbed Reduce-It, may not be available until 2016. If the FDA doesn't change its mind about the approval for the expanded indication -- which seems like a long shot to me -- three years versus five years of exclusivity is the least of Amarin's worries.

Brian Orelli has no position in any stocks mentioned, and neither does The Motley Fool. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.