Regulators are set to consider in the next two days whether Tysabri, the multiple sclerosis drug from Biogen Idec ( BIIB) and Elan ( ELN), is safe enough to return to the market. The drug was withdrawn from the market last year after three cases of a rare brain disease -- progressive multifocal leukoencephalopathy -- were linked to Tysabri. In two of the cases, the patients died. This Tuesday and Wednesday, an advisory committee to the Food and Drug Administration will convene to look at the drug's safety and effectiveness and to hear statements from patients who want the drug to be available as a treatment option. Positive data on Tysabri's effectiveness were published in the New England Journal of Medicine last week and appear to be too compelling for regulators not to at least strongly consider the drug's return. Still, concerns have surfaced that Biogen and Elan haven't adequately assessed the risk of PML that patients on Tysabri could face. The advisory committee is expected to weigh the risks and benefits of the drug and discuss its ability to stop the development of physical disabilities in MS patients. The panel will also debate whether to approve the drug's re-entry into the market with its use limited to a specific subset of patients. Additionally, the medical experts will try to determine if the drug may be used in combination with other so-called immunosuppressive agents. According to FDA briefing documents posted on the agency's Web site Monday, the committee might also request additional data or further studies on the drug. Tysabri was granted an accelerated review by the FDA on the basis of its efficacy and its potential to provide patients with a more effective MS drug than those already on the market. Advisory committee meetings usually last only one day, but the FDA extended its meeting on Tysabri to two days in order to hear from patients and the general public. Trading in shares of Biogen and Elan will be halted during the meeting. Biogen shares were lately down 32 cents, or 0.7%, at $45.18, while Elan was higher by 30 cents, or 2.4%, to $12.80.