The markets have delivered in a big way for investors in 2013, but not every stock out there has satisfied its investors. Even among big pharma stocks --many of which have posted superb years so far -- a few big names haven't lived up to the market's run. Take French pharmaceutical giant Sanofi (NYSE:SNY): This stock boasts a diverse drug portfolio and the top blockbuster diabetes therapy Lantus, which made $5 billion last year. Still, that hasn't been enough to save Sanofi's stock from a lackluster year, as shares only have posted low single-digit percentage growth in 2013.

Sanofi's facing challenges endemic to big pharma companies, particularly in regards to the patent cliff. While Sanofi has managed to ride Lantus higher in making its name as one of the most dominant firms in treating diabetes, can this top drug get this stock back on track?

This is one stock health care investors need to watch with a close eye over the near future. In the video below, Fool contributor Dan Carroll explains what has hurt Sanofi's stock in 2013 and the hurdles that this company will have to clear to get shares back on track.

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.