The Tysabri multiple sclerosis medicine, that has been produced by Irish drugs company Elan and is marketed in the US by Biogen Idec, may result in severe liver injury within six days of the first dose, according to US regulators said.

Patients are advised to to stop taking Tysabri if they develop jaundice or other symptoms of liver injury, according to a letter to doctors from the drugmakers that was posted today on the Food and Drug Administration's (FDA) web site. Doctors should warn patients about the drug's liver risk, the FDA said.

Tysabri, is both Elan's and Biogen's top selling product, generated $129 million in worldwide revenue in the fourth quarter and was being taken by 21,000 patients at year-end, according to Biogen. The companies withdrew the drug from the market in late February 2005 after two patients developed rare brain infections. The drug was relaunched in July 2006 after the FDA decided the benefits for slowing multiple sclerosis relapses outweighed the risk.

Elan's credit rating outlook has been raised to positive from stable by Moody's Investors Service, which cited steady market acceptance of Tysabri after the re-launch about 1 1/2 years ago.

Moody's also affirmed Elan's existing credit ratings, including the B3 corporate family rating.

Moody's Analyst Michael Levesque said other positive developments for the drug maker include the recent FDA approval of Tysabri in moderate-to-severe Crohn's disease, and the initiation of Phase III clinical trials of bapineuzumab in Alzheimer's disease, in collaboration with Wyeth.