2009 was a solid year for sanofi-aventis (NYSE:SNY). Thanks to a boost from sales of swine flu vaccine and some help from the exchange rate, revenue was up 6%. Cost-cutting measures helped the bottom line increase even faster -- adjusted EPS were up 18% year over year.

2010? Not so solid. Colon cancer drug Eloxatin now has generic competition from Teva Pharmaceuticals (NASDAQ:TEVA) and Hospira (NYSE:HSP) and Plavix, which Sanofi markets with Bristol-Myers Squibb (NYSE:BMY), won't see generic competition in the U.S. for a while, but generics are already on sale in many European markets.

EPS on an adjusted basis are expected to come in 2% to 5% higher. But -- and this is a big but -- that assumes no generic competition for Lovenox.

Now I realize it's difficult for Sanofi to determine if or when the FDA will approve the generic versions -- it's a complicated drug and the agency has been working on applications filed by generic-drug makers including Teva and Momenta Pharmaceuticals (NASDAQ:MNTA) for years -- but that doesn't mean investors should just ignore it either. I'd call the guidance a maximum of a 5% increase in EPS with the potential for a decrease in earnings if Sanofi loses its $4.2 billion flagship drug.

Unlike Bristol-Myers and Eli Lilly (NYSE:LLY), Sanofi is dealing with its patent cliff by diversifying away from prescription drugs and into lower-margin but more-stable businesses. It remains to be seen whether moves into emerging markets, generics, over-the-counter products, and further into animal health by acquiring Merck's (NYSE:MRK) half of their joint venture will end up saving the company in the long run.

This year however, I think it's clear that the patent cliff has the upper hand.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Momenta Pharmaceuticals is a Motley Fool Rule Breakers selection. The Fool has a disclosure policy.