Source: Biogen Idec

It's hard to imagine that Biogen Idec (NASDAQ:BIIB) believed Tecfidera would become one of the top selling multiple sclerosis drugs just one year after its launch, but that's exactly what happened.

Tecfidera was the third oral MS drug to get the FDA green-light when it was approved in March 2013, but while Novartis (NYSE:NVS), which markets Gilenya, and Sanofi (NASDAQ:SNY), which markets Aubagio, won approval first, both companies first mover advantage is disappearing quickly.

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Another great quarter
Biogen's second quarter sales jumped 40% year-over-year to $2.4 billion thanks to Tecfidera's sales surging from $192 million to $700 million, including $115 million in sales outside America.

As a result, Biogen is bumping up its full year sales growth forecast to between 38% to 41% and guiding for non-GAAP earnings per share of between $12.90 and $13.10. Exiting 2013, the company was estimating its full year sales would grow between 22% and 25%, producing non-GAAP EPS of between $11 and $11.20.

That earnings confidence stems from delivering EPS of $3.01 in the second quarter and $5.03 in the first six months, up from $2.06 and $3.85 last year, respectively.

Source: Biogen Idec

An even brighter future
More than half of the globe's multiple sclerosis patients live in Europe, and Tecfidera is only now launching in those markets.

Since Tecfidera has already displaced Novartis' Gilenya as the top selling oral MS drug in America, its likely not a stretch to assume a similar market share grab is coming in Europe as Biogen inks new pricing agreements with additional countries.

If that proves true, Tecfidera could have plenty of revenue running room over the coming year. Gilenya's sales improved 28% to $606 million in the second quarter, but that growth was driven by a 48% jump in overseas markets where it doesn't compete broadly with Tecfidera (yet).

Tecfidera could also find that it nabs business away from Sanofi's Aubagio too. Aubagio's revenue jumped by 305% year-over-year to $78 million in the first quarter after Sanofi agreed to price concessions.

Other reasons for optimism
Tecfidera is already a blockbuster and it will offer Biogen sales growth tailwinds at least through next year, but Biogen could see sales climb thanks to other new drugs too.

In March, the FDA approved Biogen's hemophilia B drug Alprolix and Biogen began marketing the drug in May. Alprolix sales totaled just $10 million in its first month or so on the market, but sales may continue to climb given Alprolix's once-weekly dosing.

The company's Plegridy, a subcutaneous pegylated interferon beta 1 drug for relapsing MS, got a positive opinion from a key European advisory committee in May that should clear the way to eventual EU approval, further strengthening the company's MS franchise.

And In June, the FDA gave Biogen's hemophilia A drug, Eloctate, a green light too. Eloctate offers a more convenient dosing schedule of once every three to five days version the current treatment regimen of three times a week or every other day. That means that Eloctate could reduce the number of patient's infusions by more than 50 a year.

Fool-worthy final thoughts
Tecfidera gives Biogen its first blockbuster MS drug since Tysabri and while the drug is already selling at annualized $2.8 billion clip, sales should march much higher as it gains stronger market share in Europe.

While Tecfidera's impact is far larger than Alprolix and Eloctate, those two hemophilia drugs help diversify Biogen's revenue away from MS. Both drugs could eventually generate hundreds of millions in annualized sales given the significant need for new treatment among the more than 142,000 people living with hemophilia A and 28,000 people living with hemophilia B worldwide.