Pfizer is buying Mylan in an all-stock deal and combining the $10bn generic pharmaceutical company with its own off-patent branded and generic business.
The Pfizer business, called Upjohn, makes Lipitor, Celebrex and Viagra.
Mylan, based just outside of Pittsburgh, is best known for its EpiPen, an injector used to halt life-threatening allergic reactions.
Pfizer shareholders will own 57% of the combined new company and Mylan shareholders will own 43%.
The deal today arrives at a precarious time for big drugmakers who are threatened by patent protection losses and lower-priced rivals.
The new company will be incorporated in Delaware, US and run operations in Pittsburgh, Shanghai and Hyderabad, India.
The combination is expected to close in the middle of next year.