Novartis AG's Gilenya multiple sclerosis drug lost market share for the first time in January.

The loss of share came after the deaths of some patients in soon after taking the drug, the first pill available for the disease in the US.

Gilenya's share of the US market for so-called immunomodulatory drugs against MS fell to 6.1 per cent from 6.2 per cent in December, according to data from market research company Wolters Kluwer NV.

The decline was the first after 15 months of growth, at a median of 15 per cent a month, since the treatment received US regulatory approval in September 2010.

The Food and Drug Administration and the European Medicines Agency are investigating 11 deaths among Gilenya patients. In the past month, analysts have cut their forecasts for peak sales of Gilenya by 10 per cent to $2.1 billion in 2016.

The deaths have "made me a little more cautious," said Aaron Miller, chief medical officer of the US National Multiple Sclerosis Society, and a medical director at Mount Sinai Hospital in New York.

"I am not somebody who has recommended Gilenya as a first-line drug prior to these reports, and I'm still not recommending it as a first-line drug until we get more data."

Mr Miller has about 800 patients under his care and has put about 30 on Gilenya since it was approved, he said. He prefers injectable drugs, which include products such as Teva Pharmaceutical Industries Ltd's Copaxone and Biogen Idec Inc's Tysabri as first-line treatments.

There have been a record number of new prescriptions in the US in the past few weeks, Switzerland-based Novartis said in an e-mailed statement.

The rate of growth may slow and then accelerate again, the company said.

"Some uncertainty is to be expected in the marketplace as some physicians and patients wait for final review and potential label changes," Novartis said.

"Overall, we see Gilenya continuing to grow in volume. Gilenya potential is seen as unchanged."

Bloomberg