6. Betaseron/Betaferon
Bayer HealthCare
2013 Revenue:
$1.38 billion 

Betaseron was Bayer's second-best-selling drug last year behind Kogenate, its orphan drug for hemophilia. But its sales are fading--off nearly 15% last year--as competition from drugs like Merck KGaA's Rebif and Biogen Idec's ($BIIB) Avonex, both beta interferons, have surpassed it. And newer oral drugs, like Biogen Idec's Tecfidera, are giving them all fits.

One reason is ease of use over injected or IV drugs, as well as fewer side effects. A recent study by AdverseEvents found that the older interferons, like Rebif, Avonex, and particularly Betaseron, are not as safe as newer entrants to the market. Betaseron had the highest reported levels of disabling or fatal side effects.

The drug goes off patent in 2016, but with sales falling, Bayer had already taken steps to cut costs. It wound down production of Betaseron at a plant in Emeryville, CA, in 2013, turning manufacturing over to Boehringer Ingelheim on a contract basis. -- Eric Palmer (email | Twitter)

For more:
Price pressure, new rivals force Bayer's MS move
Bayer to cut roughly 540 jobs in CA