Pfizer, the maker of Viagra and Lipitor, has struck a deal to buy Botox-maker Allergan in a transaction valued at about $160 billion, creating the world's biggest drugmaker.

New York-based Pfizer and Allergan, headquartered in Ireland, said on Monday that Pfizer would offer 11.3 of its own shares for each Allergan share.

Pfizer stockholders will receive one share of the combined company for each of their Pfizer shares.

The offer - the biggest ever in the healthcare sector - values Allergan's shares at $363.63 each, compared with the stock's close of $312.46 on Friday.

Pfizer chief executive Ian Read will be CEO of the combined company, while Allergan CEO Brent Saunders will be chief operating officer.  - Reuters