TRENTON, N.J. (AP) – Faced with thousands of Vioxx lawsuits, the loss of patent protection for a key product and a weak array of drugs in development, Merck & Co. is axing more than 10 percent of its work force and eliminating five production plants.

The moves, announced Monday as part of a global reorganization company officials said is designed to make Merck more efficient and competitive, drove shares down more than 4 percent.

Merck shares fell $1.42, or 4.6 percent, to close at $29.56 in heavy trading Monday on the New York Stock Exchange. Shares are down about two-thirds from their value five years ago.

The first federal Vioxx trial was to begin Tuesday in Houston. Merck has won once and lost once in state trials in New Jersey and Texas.

Richard T. Clark, who took over as CEO in May, said Merck’s revenue and legal troubles did not play a role in his strategy. He said Merck must maintain sales of its top drugs, launch new ones and better integrate late-stage research and manufacturing to reduce the time to launch new products.

“We need to execute flawlessly all of those ingredients in order to turn this around,” Clark told The Associated Press in an interview.

Merck will immediately start cutting an estimated 7,000 jobs, about half of those in the United States. The company’s manufacturing sector will take the biggest percentage of cuts _ about 60 percent _ while the rest will be spread around the company.

Supply chain and research operations also will be restructured as the company attempts to reduce pretax costs by $3.5 billion to $4 billion by the end of 2010.

By the end of 2008, Whitehouse Station-based Merck plans to close or sell five of its 31 manufacturing plants, scale down operations at some others and close three other facilities _ one research site and two preclinical development sites. The company did not specify which plants would be closed.

Analysts said Merck is following in the footsteps of other pharmaceutical companies like New York-based Pfizer Inc. and Madison-based Wyeth, which have announced restructuring and job cuts in the last year.

“This is in response to a very challenging environment,” said Morgan Stanley managing director Jami Rubin. “I would expect broader cuts to be announced within the sales force, marketing, general and (administration) as well as R&D over the longer run.”

The changes announced Monday are the latest in a series of cost-cutting efforts by Merck. Last December, then-chief executive officer Raymond V. Gilmartin announced changes aimed at cutting Merck’s costs by $2.4 billion through 2008. Merck also eliminated 5,100 jobs through buyouts and layoffs in 2003-04 and an additional 825 jobs this year.

Merck has slipped from the world’s third biggest pharmaceutical company to No. 5, by revenue, in recent years. It is facing thousands of lawsuits and tens of billions of dollars in potential liability from its recalled painkiller Vioxx, a weak pipeline of new medicines and the looming loss of patent protection next June for its blockbuster cholesterol fighter Zocor.

Zocor generates about 20 percent of Merck revenues and is the world’s second-biggest drug. But with competition from generic drugs, Merck expects Zocor sales to drop almost in half in 2006.

Analysts were split on the reasons for the cuts. Rubin blamed Zocor’s patent expiration, Vioxx litigation and not enough strong products.

Tony Butler, pharmaceutical analyst with Lehman Brothers, cited Vioxx, Zocor and an industry cost-cutting trend.

“There’s not a company that’s not swept up in this wave of cost analysis,” Butler said.

Henry L. Miller, a plaintiff’s attorney from Newtown Square, Pa., who has two Vioxx cases pending, said it would be hard to draw a connection between that litigation and the restructuring.

“If every company the size of Merck that had lawsuits filed against it went out and started closing plants, nobody would be doing any business,” Miller said.

Merck employs nearly 63,400 people, including a total of 31,000 in the United States. Vacant jobs and ones held by temporary workers will be cut first and full-time workers will get severance packages, but no buyouts are planned, Clark said.