Biogen Idec Inc. and Elan Corp.'s multiple sclerosis medicine Tysabri may harm the liver within six days of the first dose, U.S. regulators said, urging doctors to warn patients about the risk. Patients should stop taking Tysabri if they develop jaundice or other symptoms, according to a letter this month to doctors from the drugmakers and posted today on the Food and Drug Administration's Web site.  The companies said not all doctors were aware of the risk, added to prescribing information in January when the drug was approved for an expanded use. If you or someone you know has taken Tysabri, you or they may be entitled to monetary damages. Contact us for a free consultation.