The patent expirations of key drugs have hit sales hard across the pharmaceutical industry, but few companies are facing as drastic a patent cliff as Eli Lilly (NYSE:LLY) over the rest of the decade. The company has managed to avoid the worst of the patent cliff so far, but concerns about what the future holds have taken their toll on this company's stock in 2013: Eli Lilly's shares are up only around 6% year-to-date, far underperforming many of big pharma's stronger stocks over the year's course so far.

What's dragging down investor optimism? Eli Lilly will lose patent protection for its top blockbuster, antidepressant Cymbalta, later this year in a move that could cost the company top dollar on its top line. A number of other patent losses on top-selling drugs have Eli Lilly scrambling to unlock its future sales blockbusters.

Eli Lilly hasn't performed up to many of its big pharma rivals' tracks so far this year, but is there an opportunity in this stock for investors? In the video below, Fool contributor Dan Carroll tells you what's ahead for Eli Lilly -- and the big threats facing this stock in the next few years to come.

Fool contributor Dan Carroll has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.