3. Rebif
Merck KGaA
2013 Revenue:
$2.51 billion

Merck KGaA's top-selling drug, Rebif, posted slight organic growth of 1.4% last year, but thanks to foreign exchange effects of 2.9%, sales actually decreased by 1.5%.

Still, considering the new rivals Rebif's up against, that's saying something. Rebif is part of the old guard of injectable MS therapies, many of which have struggled to keep sales climbing now that there are oral treatments--or "more intense competition," as Merck KGaA calls it.

That competition is only going to get stiffer this year, Merck acknowledged in its annual report, but it's getting prepared. "Strategies for defending market share have been launched," it said. And if EvaluatePharma's predictions hold any weight, those strategies will see some success going forward: EP forecasts Rebif will still be raking in $1.76 billion in sales in 2018. -- Carly Helfand (email | Twitter)

For more:
Merck KGaA shores up Rebif defenses to fend off new MS rivals
Merck KGaA hikes forecast, eyes deals as cost cuts keep delivering
Merck KGaA sees U.S. as 'emerging market' for sales growth
Rebif news blow against Merck KGaA