The risks of Tysabri, an effective drug for treating MS, and the risks of MS patients developing progressive multifocal leukoencephalopathy the longer they are on the medication"underscores a central mathematical issue in assessing risks and benefits of medical treatment," comments the Wall Street Journal in the September 9, 2009, issue. Absolute percentages, mathematical models and actuarial calculations all figure in figuring the risks in this Biogen drug that has been on the market for three years.