If everything goes as planned, the deal will give the $45-billion Pfizer the tax benefits of an Irish domicile.
Pfizer has been desperately looking to buy Allergan after its failed attempt to buy Anglo-Swedish drug maker AstraZeneca for $118 billion.
Allergan has morphed from being an ophthalmic-focused drug firm to a serious multiproduct branded industry heavyweight through a series of deals over the past three years.
As per reports, this historic deal will involve Pfizer paying with 11.3 of its shares for each Allergan share.
Pfizer will bring together its drugs such as Lipitor, Viagra and nerve pain treatment Lyrica with Allergan's Namenda memory loss treatment, Restasis dry eye medication and other leading eye-care brands.
A Reuters report quoted many industry analysts and investors, who believed that Pfizer could be bulking up with Allergan's fast-growing brands as a prelude to splitting by 2017 into two companies - one selling high-margin branded drugs and one selling inexpensive generics that have dragged down Pfizer results over the past few years.
Allergan was founded in 1948 By Gavin Herbert and the company first marketed its first nasal drop- Allergan.
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