More problems with the multiple sclerosis med. In a letter posted on the FDA site, Elan and Biogen Idec, which co-market the drug, warned Tysabri may harm the liver within six days of the first dose.

Some patients have displayed signs that are “generally recognized as an important predictor of severe liver injury that may lead to death or the need for a liver transplant in some patients,” according to the letter. Apparently, not all docs were aware of the risk, which was added to prescribing information in January when the drug was approved for Crohn’s disease.

Biogen and Elan had pulled Tysabri from the market in February 2005 because two patients developed fatal brain infections. The drug was reintroduced in July 2006 after the FDA decided the benefits of slowing MS relapses outweighed the risks. About 21,000 patients used Tysabri, Biogen’s fastest-growing product, at the end of 2007, according to Bloomberg News.

Uncertainty over Tysabri’s prospects was one reason that Biogen failed to find a buyer recently. “This is a drug with a cloud of risk hanging over it, and each additional risk is unwelcome,” Jason Kantor, an analyst with RBC Capital Markets, tells Bloomberg. “I don’t think this will have a material impact on use of Tysabri, but if more of these things pile up it will become less compelling.”

The letter noted that some patients developed unusually high levels of liver enzymes and recurred in some patients after they received another dose, “providing evidence that Tysabri caused the injury,” according to the letter.

In July, the FDA cited 28 cases of liver injury associated with Tysabri since November 2004, four of them potentially serious. Elan spokesman Andrew Lewis said at the time the cases were from “post-marketing experience,” according to Bloomberg.

Clinical trials that compared Tysabri to a placebo found a “comparable” rate of liver injury, a Biogen spokeswoman tells Bloomberg, and the cases of liver injury have been reversible after patients stopped taking the med. None of the patients with liver damage needed a transplant or died from the side effect, she adds.

Biogen is counting on sales of Tysabri to fuel its growth. The company said this month that it expects 100,000 patients to be taking Tysabri by the end of 2010, which translates to about $2.8 billion in annual sales at current prices.