Pfizer, Allergan to merge in biggest ever healthcare deal

PFIZER, the maker of Viagra and Lipitor, has struck a deal to buy Botox-maker Allergan in a transaction valued at about $US160 billion ($A221.2 billion).

The complex deal, the biggest ever in the healthcare sector, will allow Pfizer to shift its legal base to Ireland in a so-called "inversion" that would reduce its tax rate.

Pfizer Chief Executive Ian Read will be CEO of the combined company, while Allergan CEO Brent Saunders will be Chief Operating Officer, the companies have announced.

New York-based Pfizer and Allergan, headquartered in Ireland, said