This is a very anxious week for patients afflicted with multiple sclerosis, as well as employees and investors in Elan ( ELN). On Tuesday and Wednesday, an FDA advisory committee will evaluate the safety report and clinical trial results of Tysabri, an MS drug that is a joint partnership between Elan and Biogen Idec ( BIIB).I can foresee a situation in which Elan's shares could run after this meeting, but this is a convoluted story that won't resolve easily. Tysabri is widely expected to gain approval at the end of the month and be back on the market as early as next quarter. The drug had been available for a few months in early 2005 when it was voluntarily recalled after three patients came down with a rare brain disease, and two of the patients died. The three were being treated with Tysabri and another medication, both of which suppress the immune system. The condition, PML, was not detected in any patients who were treated solely with Tysabri. However, most experts don't rule out that it could occur even if Tysabri is the only therapy given. A recent two-year study showed Tysabri was twice as effective in reducing relapses as other available therapies. There are some former Tysabri users who are demanding that they be allowed to continue with the drug. For example, MSpatientsforchoice.org was created to lobby the FDA to approve Tysabri and get it back on the market. The organization believes that the risks should be clearly defined, but that ultimately patients should be able to choose whether Tysabri is right for them. If Tysabri is approved, it will likely have a restrictive label with warnings about PML. This could limit the number of doctors who are willing to prescribe the drug due to cautiousness or even fear of lawsuits should a patient contract PML, for which there is no cure.