(AP) People taking the pain reliever Celebrex were at nearly twice the risk for heart attacks as those using rival treatments, according to a study released Wednesday conducted by New Zealand's Medical Research Institute.

Celebrex, which is manufactured by Pfizer Inc. and is commonly used to treat arthritis pain, belongs to the class of drugs known as cox-2 inhibitors. Two other drugs in the class, Pfizer's Bextra and Merck & Co's Vioxx, were removed from the market because of safety concerns. Vioxx was withdrawn in September 2004 after a study showed it doubled patients risk of heart attack and strokes after 18 months of use.

The research published Wednesday reviewed six studies of 12,780 patients in an attempt to determine if the increased risk of cardiovascular problems with Vioxx was also present with Celebrex..

It found a 1.88-fold increased risk of heart attack when Celebrex compared with the other arthritis treatments.

"These findings are critical" because Celebrex's risk is similar in magnitude to Vioxx's risk, said Prof. Richard Beasley, the Institute's director. The research was published in the Journal of the Royal Society of Medicine.

"Given the popularity of celecoxib (Celebrex) in the treatment of arthritis ... drug regulatory authorities need to urgently re-examine the assessment of the drug in light of these findings," Beasley said.

Pfizer New Zealand General Manager Mark Crotty said the finding was "extremely misleading" as it is "very much an incomplete review of the data _ selecting 6 studies out of 48 available."

"We very much dispute and are concerned with the nature of these findings. It's concerning for people who have been through a lot of uncertainty on the use of this medication and have been reassured" about its use, he said.

Pfizer said last month that Celebrex will post sales of more than $2 billion in 2006, up from $1.7 billion in 2005.

Last December the U.S.-based Cleveland Clinic announced it would lead an international study to learn whether painkillers taken for arthritis, including Celebrex, Ibuprofen and Naproxen, are safe for those at risk of heart problems.

Pfizer is slated to face its first patient lawsuit over Celebrex in June in an Alabama court where jurors will determine if the drug triggered a woman's stroke.

Pfizer shares fell 26 cents to $26.03 in afternoon trading on the New York Stock Exchange.