Final offers could value the unit at $10-12B and are due in next week.
Reckitt Benckiser (RBGLY) is the frontrunner, having become the most aggressive bidder.
"OTC assets are highly desirable for market incumbents as almost no additional infrastructure is required," says Barclays. Merck's unit would be a "good fit" geographically for Sanofi, as 70% of its revenue comes from the U.S.
Other interested parties include Bayer (BAYRY).