Pfizer is buying Mylan in an all-stock deal and combining the $10 billion 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 Inc. shareholders will own 57% of the combined new company and Mylan shareholders will own 43%.

The deal Monday arrives at a precarious time for big drug makers who are threatened by patent protection losses and lower-priced rivals. Both groups reportedly hope a merger will help reignite slowing sales growth.

The new company will be incorporated in Delaware and run operations in Pittsburgh, Shanghai and Hyderabad, India.

The combination is expected to close in the middle of next year.