Cambridge-based Biogen Idec Inc. and Ireland-based Elan Corp. defended their multiple sclerosis drug Tysabri after reporting two new cases of a potentially fatal side effect, saying the treatment is still worth the risk to patients.

There are no plans to pull the drug off the market, and the company said its current monitoring program for cases of a rare but sometimes fatal brain disorder is adequate.

The announcement of two new cases of the rare viral infection called progressive multifocal leukoencephalopathy, or PML, sent shares for both companies plummeting and reignited already simmering concerns about the drug’s sales potential.

It was pulled from the market in 2005 after being linked to the rare brain disease but was reintroduced under restricted sales conditions in mid-2006. The risks associated with the drug are clearly labeled, the company said, and all patients are not only notified but have to sign a waiver acknowledging the risks.

PML almost always occurs in people with a severe immune deficiency, as is the case with most patients taking Tysabri. Multiple sclerosis is an autoimmune disorder that results in physical and neurological damage.

There are about 32,000 patients on Tysabri worldwide, with 17,800 in the U.S., the company said. It would not comment on its financial outlook, but did say future cases of Tysabri were anticipated and factored into the goal of eventually getting the drug to 100,000 patients worldwide.

Biogen and Elan would not comment the number of suspected cases, only confirmed cases. Both new cases came out of European, with the first patient having been on Tysabri for 17 months. Another, who is still hospitalized, had been on Tysabri for 14 months.

Still, the company said it can count about 6,600 patients on the drug for about 18 months now, reinforcing its stance that the treatment is worth the risk.