There are thousands of drugs on the market, and there are thousands more that are no longer in use. However, none has been more successful in terms of sales than AbbVie's (ABBV -0.41%) Humira. That's why investors were rightly concerned when the immunology medicine ran out of patent exclusivity in the U.S. last year. AbbVie's CEO, Rick Gonzalez, called it the "largest loss of exclusivity event" in the history of the industry.However, AbbVie saw this headwind coming from a mile away. Though its sales are dec ...