Pfizer (NYSE:PFE), the world's largest pharmaceutical company, has performed well in recent years despite worries about the patent cliff. Of course, losing market share for the world's best-selling drug of all time to generic competition can lead to such concerns. Management was well prepared and had two potential blockbuster drugs -- both projected to reach peak sales of more than $2 billion -- ready to launch in 2013. JAK inhibitor Xeljanz was approved for treating rheumatoid arthritis last November, and blood thinner Eliquis, developed with Bristol-Myers Squibb (NYSE:BMY), was approved one month later for reducing risk of blood clots in patients with non-valvular atrial fibrillation. Investors could sit back and ride out a successful year, right?

Unfortunately, neither drug has started commercial life in the first six months of 2013 with a bang. Xeljanz recorded revenue of just $33 million while Eliquis notched only $34 million for Bristol (Pfizer has not disclosed Eliquis sales this year) during the period. Why are these once-heralded drugs faltering? Why aren't their advantages playing out as envisioned? Fool contributor Maxx Chatsko breaks it down for investors in the following video.

Fool contributor Maxx Chatsko has no position in any stocks mentioned. Check out his personal portfolio, his CAPS page, or follow him on Twitter @BlacknGoldFool to keep up with his writing on energy, bioprocessing, and biotechnology.

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