Biogen Idec's fourth-quarter net profit fell slightly due to a tax accounting error.

But the company's shares rose 3% on optimism over BG-12, a new treatment for multiple sclerosis that Biogen expects to introduce in the second quarter.

BG-12, an oral drug to be sold under the brand name Tecfidera, is expected to become a leading treatment for the disease. Analysts on average are expecting sales of Tecfidera in 2013 of about $350m.

In the fourth quarter, global sales of Biogen's multiple sclerosis drug Tysabri rose 14% to $433m from $380m a year ago, while sales of Avonex rose to $753m from $703m. Biogen markets Tysabri in conjunction with Elan.

"We believe we are entering into an era of significant long-term growth driven by multiple potential new product launches," George Scangos, Biogen's chief executive officer, said today.

The company said its fourth quarter net earnings were $292m, or $1.23 per share, compared with $300m, or $1.22 a share a year ago. Revenue rose to $1.4 billion from $1.3 billion a year ago.

The company said that an error in its tax accounting related to its Denmark facility accumulated over several years had hurt earnings by an unexpected 12 cents per share.

Biogen said it expects 2013 earnings per share excluding one-time items to be between $7.15 to $7.25 per share and revenue to grow 10% from $5.5 billion in 2012.