Last week, French drugmaker Sanofi-Aventis (NYSE:SNY) announced first-quarter financial results, with sales up almost 7% on a constant-currency basis and adjusted net income down 3%. However, the company's performance in 2007 will be dictated more by major legal decisions over its two top patented compounds, the anticoagulants Lovenox and Plavix, coming later this year.

Sanofi already experienced setbacks with Plavix last year and Lovenox this year. Despite these setbacks, it raised earnings guidance for 2007 on the big assumption that it won't face generic competition on either of these compounds this year. (Generic rivals such as Watson Pharmaceuticals (NYSE:WPI) disagree.)

The EPS growth estimate for the year increased three percentage points for 2007, from 6% to 9%, versus Sanofi's adjusted EPS of $6.10 (at an exchange rate of $1.25 per euro) for 2006. Even if Sanofi prevails in court over Plavix and Lovenox, changes in currency rates and the possibility of a weakening dollar will likely drive this guidance downward.

Shares of Sanofi may not look expensive on the surface, but there's a lot of risk here. In addition to the Plavix and Lovenox concerns, more than 70% of the revenue from its Ambien franchise is about to feel the wrath of generic competition. If you jump into shares of Sanofi today, you'll be taking a major bet on an uncertain near-term future.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.