Merck's off-patent drugs are called "diversified brands" and many are sold in emerging markets.
The sale processes underscore efforts by large drugmakers to shed smaller divisions they view as non-core so they can better focus on their mainstay products. They have shown new willingness to consider large asset swaps with rivals to exit weaker businesses and bolster core areas where they are already top players.
Sanofi, being advised by Evercore Partners, is also in the market with its aging drug portfolio, which could fetch between $7 billion and $8 billion, Reuters reported on Tuesday.
Representatives for Merck declined to comment. The sources asked not to be named because the matter is not public.
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Both Merck and Sanofi have an extensive line-up of older products that could be carved out, following similar moves now under way at Pfizer and GlaxoSmithKline to place older products in separate divisions.
Pfizer announced last year it planned to separate its business into three units - innovative pharmaceuticals; vaccines, oncology and consumer health; and established products. The U.S. group has not ruled out a full breakup.
GSK is starting down a similar path, and CEO Andrew Witty said on Wednesday that he would consider single products or broader divestiture of its established drugs.