New painkillers in the same class as Vioxx that increase the risk of stroke and heart attack shouldn't be approved if safer alternatives are available, according to federal documents released Tuesday.

That opinion came as a panel of government advisers prepares to weigh Merck & Co. Inc.'s proposed successor to its now-withdrawn Vioxx.

That puts the cardiovascular safety of the drug, called Arcoxia, front and center Thursday, when the panel of Food and Drug Administration advisers discusses whether to make a nonbinding recommendation that the prescription painkiller receive agency approval. Merck voluntarily pulled Vioxx from the market in 2004 after it was linked to a higher risk of stroke and heart attack when compared to dummy pills.

The FDA said its focus in evaluating Merck's application for Arcoxia, and all other drugs from that class, will be specifically on its risks to the cardiovascular system. Any nonsteroidal anti-inflammatory drugs, or NSAIDs, merit approval only if they fill an unmet need for a particular group of patients who have no relatively safer options available, the FDA said in a March 21 memorandum released Tuesday.

Arcoxia and Vioxx are types of NSAIDs called Cox-2 inhibitors, developed to be gentler on the stomach. The only Cox-2 inhibitor still sold in the U.S. is Pfizer Inc.'s Celebrex. The New York company withdrew another of the drugs, Bextra, in 2005.

Merck wants FDA approval to sell Arcoxia, also known as etoricoxib, to treat the signs and symptoms of osteoarthritis. The Whitehouse Station, N.J. company said its drug should be an option for the estimated 21 million Americans who suffer from osteoarthritis.

The company said the cardiovascular risk of Arcoxia was comparable to another, older NSAID called diclofenac, based on the results of a study that enrolled nearly 35,000 patients. Critics have said a fairer comparison would have been to use other NSAIDs, like naproxen, sold as Aleve and under other names. They maintain diclofenac raises heart risks. Merck disputes that.

The FDA, meanwhile, said results from smaller and shorter studies suggested the cardiovascular risk of Arcoxia was greater than that posed by either naproxen or dummy pills, but less that of other NSAIDs.

Merck already sells Arcoxia in more than 60 other countries. The company expects a final FDA decision on the drug as early as by month's end.

The safety of Cox-2 inhibitors has drawn intense scrutiny from regulators, drug companies, academics, lawmakers, advocacy groups and the media in the wake of Vioxx's withdrawal.