Federal regulators will debate the potential risks of letting a multiple sclerosis drug back on the shelves. An advisory panel will spend Tuesday and Wednesday deciding whether Tysabri should be back on the market.

Drug makers voluntarily took it off the market in February 2005 after three users developed a rare and often fatal brain disease. Two of them died. The two companies want to begin selling it again, saying its benefits outweigh the risks. They're also proposing a plan to guard against more cases of the brain disease.