The popular pain-killers Celebrex and Vioxx may slightly increase a person's risk of having a heart attack, according to an analysis of clinical studies of these 'super aspirin' drugs.

"It is a cautionary flag that seems to say that something is going on that needs further exploration," said Eric J Topol, a cardiologist at the Cleveland Clinic and one of the authors of the analysis, which appeared in yesterday's journal of the American Medical Association.

Since their introduction in 1999, the drugs, which are known technically as COX-2 inhibitors, have been among America's best-selling prescription pharmaceuticals. One of them, Celecoxib, sold as Celebrex, was the sixth leading prescription drug used by 14 million people last year. The other COX-2 inhibitor is Rofecoxib, sold as Vioxx.

The drugs are popular because they provide the same pain- killing effects as aspirin and Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) such as ibuprofen, but with less chance of causing ulcers and intestinal bleeding.

Aspirin inhibits two closely related enzymes, COX-1 and COX-2. The new drugs specifically inhibit only the second (which is involved in pain and inflammation) while not affecting the first (which, among other things, is involved in protecting lining of stomach).