The U.S. Food and Drug Administration warned patients of the risk of heart damage from Serono’s multiple sclerosis drug Novantrone, a niche product, weighing on the Swiss company’s stock on Wednesday.

“Reports received through post-marketing surveillance have shown that diminished cardiac function may occur early on in the treatment with Novantrone,” Serono told.

It added that it had updated the product labeling for the drug.

The FDA said congestive heart failure, a potentially fatal condition, could occur either during treatment with Novantrone, or months to years after termination of therapy.

The updated label also included information about an increased risk of developing treatment-related leukemia.

“In 1,774 patients with breast cancer who received Novantrone concomitantly with other cytotoxic agents and radiotherapy, the cumulative risk of developing treatment-related AML (acute myelogenous leukemia), was estimated as 1.1 percent and 1.6 percent at five and 10 years, respectively,” Serono said in the statement on the Web site.

Industry analysts said the news would have only a limited financial impact on Serono, since Novantrone is a last-resort treatment that generates only modest sales.

“The drug is burdensome already because it has a large side effect catalog,” said Lombard Odier Darier Hentsch analyst Karl-Heinz Koch.

“It is only used in very severe cases anyway. I don’t believe it will have a big effect on Serono or a large impact on the use of the drug,” he added.

Novantrone is one of Serono’s multiple sclerosis products, which the firm sells besides its flagship Rebif multiple sclerosis drug.

The drug had sales of $83.9 million in 2004, representing 3.9 percent of total product sales, and around full sales potential, according to a Serono spokesman.

“We believe it is not going to have an effect on the product. The product is for very severe forms of MS, for people who have no choice,” the spokesman said.

Analysts at stockbroker Jefferies in London said sales of Novantrone had failed to meet expectations since Serono licensed the drug from Amgen two years ago and it forecast sales would fall 17 percent this year to $70 million.