Sanofi shares were down 2.3 percent in early session trading, making the stock the worst performer on France's benchmark CAC-40 index.
Sanofi also underperformed a flat performance on the STOXX Europe 600 Healthcare index.
Sanofi has agreed to buy all of the outstanding shares of Bioverativ for $105 per share in cash, marking a premium of 64 percent to Bioverativ's closing price on January 19.
"In that sector, you don’t get anything cheap. You’ve got to pay up. People do appear to be a bit sceptical on the price, but not overtly so," said Terry Torrison, managing director at Monaco-based McLaren Securities.
Prime Partners' fund manager Francois Savary took a more upbeat view of the transaction.
"In my view, it's essentially a positive deal. It should strengthen their capacity to grow in the medium term," said Savary, whose firm owns Sanofi shares.
(Reporting by Sudip Kar-Gupta; Editing by Laurence Frost)