A government-sponsored trial of Pfizer Inc.‘s blockbuster arthritis drug Celebrex for preventing tumors was halted after patients taking the medicine had more than twice as many heart attacks as patients taking a placebo, the company said on Friday.

Shares of Pfizer, a component of the Dow Jones industrial average, fell 17 percent in early trading following news of the long-term trial, sponsored by the National Cancer Institute.

The heart-attack findings for Celebrex come only 10 weeks following Merck & Co.‘s recall of its similar arthritis drug, Vioxx, after a study found the drug doubled the incidence of heart attack and stroke among patients taking it to prevent colon polyps that cause cancer. Both drugs are from a class known as COX-2 inhibitors.

“This does not bode well for COX-2 inhibitors in general,” said Ira Loss, an analyst at Washington Analysis. “The sense had been that Celebrex is somehow different from the others.”

A Pfizer spokesman declined to say if the company was considering pulling Celebrex off the market.

Pfizer said the trial involved patients taking 400-milligram and 800-milligram daily doses of Celebrex to prevent adenomas, tumors that grow from glandular tissue. The anti-inflammatory drug was being tested on the theory that inflammation is a cause of cancer.

Officials of the U.S. Food and Drug Administration were not immediately available for comment.

Vioxx and Celebrex both work by selectively blocking a protein called COX-2 that has been linked to inflammation. They were both launched in 1999 and quickly became top-selling drugs, helped by massive television and print advertising.

Pfizer also said on Friday that Celebrex was not shown to increase heart risk in a second long-term trial designed to see if the drug could prevent colon polyps.

The New York-based drugmaker said National Cancer Institute officials decided to halt the other Celebrex trial, to test its ability to prevent adenomas, after confirming “an approximately 2.5-fold increase in their risk of experiencing a major fatal or non-fatal cardiovascular event compared to those taking placebo.”

“They (the NCI) will continue to follow patients in the study,” said Pfizer spokesman Paul Fitzhenry, although patients will no longer be given Celebrex.

“We alerted the Food and Drug Administration last night about the findings,” said Fitzhenry. He said he could not elaborate on those discussions.

Asked if Pfizer was considering pulling Celebrex off the market in view of the findings, Fitzhenry said, “We cannot comment beyond our earlier statement,” which did not discuss the possibility of withdrawing the drug.