The world's biggest drugmaker Pfizer is set to cut 18 per cent of its global workforce - a loss of 6,000 jobs - as it plans to streamline operations worldwide following its acquisition of major rival Wyeth last year.

The Manhattan-based company will halt operations at eight plants in the US and Ireland by 2015 and reduce activities at six factories in these countries, in addition to Britain and Germany.

The $67bn takeover of Wyeth last October has added more than 36 manufacturing facilities to Pfizer's 40 factories worldwide, and a total of 20,000 jobs are expected to be cut from their combined workforce.

"The restructuring of our global plant network is critical to our efforts to remain competitive", said Nat Ricciardi, Pfizer's global manufacturing president.

Wyeth, known for its oral contraceptives, brought $5bn in revenue to Viagra manufacturer Pfizer during the first quarter.