Shares of Merck & Co. (MRK) crept higher Friday after collapsing a day earlier when the company announced it would pull its blockbuster pain reliever Vioxx (search) after a study found the drug doubled patients' risk of heart attack and strokes when used long-term.

Merck stock rose 58 cents, not quite 2 percent, to $33.58 on the New York Stock Exchange (search).

On Thursday, Merck shares plunged $12.07, nearly 27 percent, to $33 — its lowest close in eight years. That wiped out $28 billion in market value. More than 140 million shares were traded, compared to a daily average below 10 million.

Also Friday, Merck raised its estimate of the number of Vioxx prescriptions written in the United States.

Merck said about 105 million U.S. prescriptions were written for the drug from May 1999 through August 2004. The drug giant said it has not determined the number of prescriptions written outside of the United States.

Previously, Merck said the drug had been prescribed about 84 million times since 1999.

The company said that, based on the revised numbers, about 20 million people have taken Vioxx in the United States since its launch in 1999.

Merck said it is withdrawing the drug following data from a new three-year trial of Vioxx, designed to evaluate the effectiveness of the drug's standard 25 milligram dose in preventing recurrence of colorectal polyps. Such polyps often become cancerous.

"In this study, there was an increased relative risk for confirmed cardiovascular events, such as heart attack and stroke, beginning after 18 months of treatment in the patients taking Vioxx compared to those taking placebo," Merck said in a release.

Vioxx, used by two million people around the world, accounts for 10 percent of Merck's annual sales.

A recent study by the Food and Drug Administration (search) suggested patients taking Vioxx faced a 50 percent greater risk of heart attacks and sudden cardiac death than those taking Pfizer Inc.'s (PFE) rival Celebrex (search) treatment.

The withdrawal of the drug casts a cloud over an entire class of widely used arthritis and pain drugs known as COX-2 inhibitors (search). "This has implications for all members of this class," said Dr. Garret FitzGerald, chairman of the Department of Pharmacology at the University of Pennsylvania.

Merck said that in a colon cancer trial, patients who took Vioxx for three years faced twice the risk of cardiovascular events, such as heart attack and stroke, as patients taking a placebo.

"Patients who are currently taking Vioxx should contact their health care providers to discuss discontinuing use of Vioxx and possible alternative treatments," it said.

Concerns over the drug's side effects have been building in recent years after several studies showed risks attached to it. Other drugs in the same class, including Celebrex and Bextra and Novartis AG's Prexige, have so far not shown the same dangers.

"This is a very significant negative for Merck. Not only is this a nearly $3 billion drug, but it calls into question the future of one the key drugs in its pipeline, Arcoxia," said Scott Henry, an analyst at Oppenheimer & Co.

Arcoxia (search), which is similar to Vioxx, is sold outside the United States but has not yet been approved by the FDA because of concerns about heart and stroke risk. Some analysts had expected the agency to rule on Arcoxia by late October.

Merck is already struggling with slowing earnings growth and faces the loss of patent protection for its biggest-selling drug, cholesterol fighter Zocor, in 2006.

Despite the setback, Merck Chairman and Chief Executive Raymond Gilmartin said he had no intention of resigning.

Reuters and the Associated Press contributed to this report.