Drugmaker Pfizer Inc will cut 6,000, or 18 percent, of jobs at its 78 manufacturing plants over the next five years as it consolidates operations following last year's purchase of smaller rival Wyeth.

The world's biggest drugmaker said it would cease operations at eight plants in Ireland, Puerto Rico and the United States by late 2015 and reduce activities at six factories in those countries, Germany and Britain.

Pfizer had 40 manufacturing sites before acquiring more than three dozen Wyeth facilities in the October merger.

The affected plants make conventional pills, injectable medicines, biotech drugs and consumer healthcare products. Pfizer will wind down their operations over the next 18 months to five years, depending on business considerations such as the time required to transfer product manufacturing.

The company had said in November that it would close six research sites and trim jobs in the United States and Britain as part of its ongoing absorption of Wyeth.

We have a complex network of manufacturing plants, with excess capacity that is not good for costs, Nat Ricciardi, Pfizer's president of manufacturing, said in an interview.

Pfizer can be more competitive, both in its operations and drug pricing, by streamlining its plants and improving their processes, Ricciardi said.

It's not disproportionately Wyeth, Ricciardi said, adding that many legacy Pfizer plants and employees are on the target list.

One of the biggest incentives for companies to merge is the ability to cut overlapping operations and employees, creating huge cost savings.

Pfizer is counting on savings from the Wyeth merger to help offset expected plunging sales of its $12 billion-a-year Lipitor cholesterol fighter, which begins facing generic competition late next year.

The 14 plants slated to cease operations or downsize are among Pfizer's 40 main sites for making prescription drugs.

The company plans by June to make recommendations for facilities that make animal-health products and later this year will evaluate those that make drugs in emerging markets or nutritional products.

We're not announcing closures, we're announcing exits, Ricciardi said, citing hopes that Pfizer will be able to sell some of the plants to owners that would continue to operate them.

Although affected employees will have as long as five years to look for other jobs, Ricciardi predicted relatively few of the workforce cuts would come from attrition, given the attractiveness and good pay of the positions.

Pfizer plans to discontinue operations over the next 18 months to five years at three sites that make tablets and capsules: Caguas, Puerto Rico; Loughbeg, Ireland and Rouses Point, New York.

Another such facility will be phased out at Guayama, Puerto Rico, although it will expand its consumer healthcare operations. The company expects to reduce the size of two others, in Illertissen, Germany and Newbridge, Ireland.

Two plants that make sterile injectable drugs -- in Dublin, Ireland, and Carolina, Puerto Rico -- are also ending operations.

(Reporting by Ransdell Pierson; Editing by Lisa Von Ahn)