Shares of Idec and Elan Corp. Plc fell 57 and 40% respectively after the death of one patient who used the new multiple sclerosis drug Tysabri of their manufacturing. The patient died from a rare and often fatal central nervous system disease.

The companies have suspended the dosing in all clinical trials of the drug.

The approved drug was expected to become the world’s leading multiple sclerosis treatment in the race with Elan and Avonex of the Biogen Idec manufacturing, that is now on top of the market.

"If the drug comes back it will have a huge safety cloud over it and will require a set of investigations of all patients pre-and post marketing approval," said Geoff Porges, an analyst with Bernstein & Co. "That is likely to take months, not days or weeks."

"The largest blow is, of course, for Elan, as they came back sharply on Tysabri after all the setbacks in recent years," said Bob Pooler, an analyst at Swiss brokerage Lombard Odier Darier Hentsch.