The FDA issued a warning last week that the multiple sclerosis drug Avonex, made by Biogen Idec, might cause severe liver damage in rare cases.

Avonex has been on the market since 1996 and is used by approximately 130,000 patients. Biogen posted a statement on the FDA website stating that severe liver injury, including liver failure, has been reported rarely in patients taking Avonex. The company advised doctors to use caution with patients who use alcohol or are taking other drugs that can damage the liver.

The "precaution" that currently appears on the Avonex label for liver damage will be upgraded to a "warning."

Avonex is Biogen's largest selling drug, with sales of $1.4 billion in 2004.

Two other drugs to treat multiple sclerosis have had troubles recently, due to a link with a rare brain infection. Tysabri, also manufactured by Biogen, was taken off the market in February. And the FDA recently ordered GlaxoSmithKline to halt a trial of a multiple sclerosis drug in the same class as Tysabri.

Tysabri interacts with white blood cells to prevent them from attacking the nerve cells, but this interaction also can create a vulnerability for brain infection. The FDA approved Tysabri for the market last fall and Glaxo was in Phase 2 testing of its drug. The Glaxo drug has a different chemical composition than Tysabri, but acts on the same protein.

The FDA acted when two clinical trial patients who had been taking Tysabri and Avonex for more than 2 years were diagnosed with brain infections, from which one patient died.

Tysabri will stay off the market while Biogen reviews the medical records of the patients in clinical trials and to administer additional testing on those patients.

The New York Times cites a British database, Pharmaprojects, which reports that there are 12 drugs in the class of Tysabri currently in trials. Most of those drugs are being tested for treatment of asthma, but five are being tested for multiple sclerosis.