Investors in Irish biotech Elan (NYSE:ELN) are probably hoping that the second time's the charm. After some extensive work, the company and its partner BiogenIdec (NASDAQ:BIIB) have resubmitted their troubled multiple sclerosis drug Tysabri for FDA approval.

The two companies received a priority review designation from the FDA. As a result, their application will be reviewed more quickly, potentially shaving as much as four months off the normal time. The reason why the companies got this priority review for Tysabri shouldn't be ignored: The drug satisfies a significant unmet clinical need.

Readers without direct experience with multiple sclerosis may not appreciate just how debilitating it is, or the impact it has upon people's lives. There are few good options available for treatment, and patients don't always respond to them especially well. And for whatever safety problems there might be with Tysabri, the drug was quite effective for some patients.

We at The Motley Fool have talked a lot about Tysabri and the safety issues, and there's no need to rehash that. (The drug has been linked to a few cases of the rare neurological disease PML, or progressive multifocal leukoencephalopathy.) The question now is how the FDA will respond. Will the companies' newly submitted data, safety assessment, and revised labeling be good enough for approval?

It's a tough call. On one hand, I'm sure the FDA wants to be sensitive to the needs of some very sick individuals, many of whom would likely be willing to assume whatever slight (but serious) risks go with Tysabri in exchange for the possibility of a better quality of life. On the other hand, the FDA has been a tough customer of late with some other drugs -- potentially holding upBristol-Myers' (NYSE:BMY) experimental diabetes drug Pargluva for years by requesting more data.

FDA approval is now the name of the game. Even if the FDA slaps a serious black-box warning label on the drug, once it's approved, doctors have considerable latitude in how they prescribe it for patients. And considering that Tysabri can cost more than $20,000 per patient per year (and there are about 350,000 MS patients in the US), even capturing 10% of the market would be a very meaningful opportunity for Elan.

I said before that I thought the safety concerns would take away Tysabri's chance to be a blockbuster (generally defined as $1 billion in sales). Maybe I'll be right, maybe I'll be wrong. I'd be more than happy to write a follow-up piece a year from now saying I was wrong to doubt Tysabri's potential -- not because it means that Elan shareholders made money, but because it would mean that a lot of patients are seeing some real improvement in their lives. In any case, Elan shareholders should buckle their seatbelts. The next few months are apt to be a heckuva ride.

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Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).