A little less than four years ago, Elan (NYSE:ELN) and partner Biogen Idec (NASDAQ:BIIB) received approval to market their innovative multiple sclerosis treatment Tysabri. Three months later, in February 2005, the drugmakers "voluntarily" withdrew Tysabri from the market because of two occurrences of a rare and frequently deadly brain disorder in patients taking the drug.

Tysabri eventually returned to the market in the middle of 2006 with a plan in place to minimize risk. Flash-forward to a couple of weeks ago, and Elan and Biogen Idec reported two more occurrences of the rare brain disorder, called progressive multifocal leukoencephalopathy (PML), in patients taking Tysabri.

On Monday, the Food and Drug Administration posted an alert on its website (seen here) to health-care professionals in response to the new cases of PML and put to rest any worries that Tysabri would be pulled from the market again. The FDA also said it was working to "amend the product labeling" for Tysabri to account for the new cases of PML.

When the two cases of PML were diagnosed in Tysabri patients in 2005, it was in patients taking the drug in combination with Biogen Idec's other multiple sclerosis treatment, Avonex. A third case of PML associated with Tysabri was discovered later as well. The FDA, Biogen Idec, and Elan then took action to help restrict Tysabri from being used with other drugs that affect the immune system, like Avonex, Rebif from Pfizer (NYSE:PFE), or Remicade from Johnson & Johnson (NYSE:JNJ). Tysabri was brought back onto the market.

But the two new cases of PML that arose last month were in multiple sclerosis patients who were not taking any other drugs with Tysabri. With this new development, the worry was that the FDA might take drastic further steps to restrict Tysabri's use or take it off the market again, as it did in 2005.

Tysabri's label already indicates that it's recommended for use in patients failing with other commonly used multiple sclerosis therapies like Teva's (NASDAQ:TEVA) Copaxone or Bayer's Betaseron. Undoubtedly, there won't be as much demand for Tysabri with these two new safety issues, and it's now even more unlikely that Biogen Idec will hit its target of "100,000 patients"  taking Tysabri by the end of 2010. But investors can now rest a little easier knowing that the FDA isn't handling the new adverse events with Tysabri the same way that it did in 2005.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.