Allergan shareholders would receive 11.3 shares in the combined entity for each of their shares.
Pfizer stockholders can get cash or one share of the combined company for each of their shares, but the aggregate amount of cash must range from $6 billion to $12 billion.
Plans call for four current directors of Allergan, including Saunders and Executive Chairman Paul Bisaro, to join Pfizer’s 11-member board, the companies said.
Reports that the companies were in talks emerged a month ago.
Pfizer was advised by Guggenheim Securities, Goldman Sachs & Co, Centerview Partners and Moelis & Co. Its legal advisers are Wachtell, Lipton, Rosen & Katz; Skadden, Arps, Slate, Meagher & Flom LLP and A & L Goodbody.
Allergan was advised by J.P. Morgan, Morgan Stanley and Cleary Gottlieb Steen & Hamilton LLP. Latham & Watkins LLP and Arthur Cox are its legal advisers.