This follows the decision of European regulators two weeks ago advising against the use of Tysabri for Crohn's disease, saying the risk of serious infection outweighed relatively modest benefits from the drug.

Tysabri is produced by Ireland's largest pharmaceutical manuafctutrer, Elan, in partnership with Massachusettsbased Biogen Idec. Both firms are currently appealing the European decision.

Tysabri was returned to the market in July 2006 as a treatment for multiple sclerosis after a 17-month suspension because of its link to a rare, fatal brain disorder.

The FDA cleared Tysabri for sale based on studies showing it was more effective than existing therapies. The FDA also ordered strict safety measures to monitor for the dangerous side effect.

Biogen reported second quarter revenues of $773m last week, a 17% increase on $660m last year. Sales of Tysabri totaled $72m in the quarter, shared between Biogen and Elan, with Biogen accounts showing the US firm netted $48m. Elan said 14,000 patients used the drug and its second quarter revenue from Tysabri was almost $47m.

Elan posted six-month losses of almost 103m last week with a 38m accounting charge affecting earnings.

Elan's revenue rose 38% to 137.5million.