Biogen Idec Inc reported third-quarter earnings on Tuesday that topped expectations, but data related to its controversial multiple sclerosis drug was mixed and its shares fell 1.5 percent.

Revenue from sales of Tysabri rose 21 percent to $207 million, but there was a decline in the weekly number of new patients added to the rolls.

According to Geoffrey Meacham, an analyst at J.P. Morgan, only 223 net new patients were added per week in the latest quarter, down from 262 a week in the second quarter.

Overall, we thought the quarter looked okay on a revenue basis for Tysabri, but lower new adds were a negative surprise, said Meacham.

On a conference call with investors, Biogen said it has determined that the risk for patients of developing a potentially deadly brain infection known as PML increases with the number of Tysabri infusions received.

The company said that together with partner Elan Corp Plc, it is discussing the possibility of changing the drug's label to reflect the increased risk of PML with duration of treatment.

The Cambridge, Massachusetts-based biotech company said net profit rose 34 percent to $278 million, or 95 cents a share, from $207 million, or 70 cents a share, a year earlier. Revenue rose 3 percent to $1.12 billion.

Excluding one-time items, the company earned $1.12 a share. Analysts were on average expecting $1.04, according to Thomson Reuters I/B/E/S.

Revenue from Avonex, the company's main product for treating multiple sclerosis, rose 1 percent to $580 million. U.S. sales of the drug rose 8 percent to $348 million.

The company said its board had authorized a $1 billion share repurchase program. It said this was in addition to the 6 million shares remaining from its previous share repurchase authorization.

While we have used the prior program principally for share stabilization, this new $1 billion authorization is intended to reduce our shares outstanding with the objective of returning excess cash to shareholders, the company said.

Biogen said it expects to report 2009 revenue in the mid to high single digit range. It expects earnings excluding one-time items to be above $3.85 a share and net earnings to be above $2.97 a share.

Biogen shares were down 1.5 percent at $48.25 in trading before the market opened.

(Reporting by Toni Clarke; Editing by Lisa Von Ahn, Dave Zimmerman)