Ailment: Rheumatoid Arthritis
2014 U.S. Sales: 6.5 billion
Humira has been a blockbuster drug for AbbVie since its approval by the FDA in 2002, but competitor Amgen has is testing a cheaper, generic form of the medication that could hit the market as early as 2017. That could be bad news for AbbVie, which gets more than 60 percent of its revenue from Humira.