5. Tysabri 
Biogen Idec
2013 Revenue:
$1.41 billion

Tysabri revenues got a spike last year from a deal between Biogen Idec ($BIIB) and Elan ($ELN), in which the Massachusetts biotech forked over $3.25 million to buy full control of the drug from its partner. That agreement, combined with price hikes, powered a top-line jump of 34% between 2012 and 2013.

Outside the U.S., though, revenues shrank from $752.8 million to $712.3 million, a drop the company attributed partially to price reductions from some countries' austerity measures. A decrease in demand--particularly in Europe--also took its toll.

Unit sales volume dropped in the U.S., too, and for that Biogen has its own Tecfidera to thank. The new therapy--the third MS pill on the market--took off running, soon eclipsing its oral competitors and grabbing market share from its injectable rivals, too.

But unlike Avonex, Biogen's top MS seller, Tysabri isn't just indicated for the central nervous system disease. It also has the FDA's OK to treat adult patients with Crohn's disease who have had inadequate response to--or can't tolerate--more conventional therapies for the disease and TNF inhibitors. -- Carly Helfand (email | Twitter)

For more:
Biogen buys control of Tysabri, leaving Elan in search of a future
Tysabri label change could add $1B to sales
FDA: Tysabri risks greatest during year 3