TRENTON, N.J. ? Vioxx, the blockbuster arthritis drug heavily promoted on TV and taken by tens of millions of people, was pulled from the market by its maker Thursday after a study found it doubled the risk of heart attacks and strokes.

Experts advised patients to immediately stop taking Vioxx and talk to their doctors about alternatives.

“Given the availability of alternative therapies, and the questions raised by the data, we concluded that a voluntary withdrawal is the responsible course to take,” said Ray V. Gilmartin, chairman, president and chief executive of Merck & Co.

The news of Vioxx’s dangers came five years after Merck put the drug on the market with great fanfare. Vioxx has since become one of the world’s most aggressively marketed drugs.

The withdrawal marks a serious blow to New Jersey-based Merck, the world’s third-largest drug maker. Vioxx accounted for $2.5 billion in worldwide sales in 2003, and has been taken by 84 million people worldwide since its introduction. An estimated 2 million people are now using it.

Merck stock plunged $12.07, or nearly 27 percent, to close at $33 in extremely heavy trading, wiping out $28 billion in market value. Merck also dragged down the Dow Jones industrial average, which was off by more than 55 points.

Vioxx, which also is prescribed for acute pain and disorders such as carpal tunnel syndrome, is seen as a potential cancer-prevention medicine as well. In fact, the recall was prompted by a three-year study aimed at showing the drug could prevent the recurrence of potentially cancerous polyps in the colon and rectum.

Participants taking Vioxx for more than 18 months were found to be twice as likely as those given dummy pills to suffer a heart attack, stroke or other heart complications.

Personal-injury lawyers already have begun circling Merck, with at least one announcing plans for a class-action lawsuit. Another claimed he already represents 58 patients across the country allegedly harmed by Vioxx, including people who suffered a heart attack, stroke, internal bleeding or kidney failure.