The news from France that the New York-based drugmaker signed a ‘pre-merger’ deal with Sanofi-Aventis is pumping the stock. Shares are up more than 6 percent this morning.

Sanofi-s ceo wouldn’t say anything. “I have no comment,” Jean-Francois Dehecq told reporters on arriving for a meeting about pharmaceuticals counterfeiting in Geneva.

In a research report issued this morning, Tim Anderson, a Prudential Equity analyst, notes a couple of caveats to keep in mind: “The problem with a deal like this is that…each company, by our math, has roughly 30% of total revenues disappearing” between 2010 and 2012 thanks to patent expirations.

As an example, Sanofi’s second-biggest drug, Lovenox, could face generic competition in the U.S. this year. He calls this sort of issue an “unappreciated risk.”

[tags]Bristol-Myers Squibb, Merger, Sanofi-Aventis[/tags]