A drug designed to relieve pain and inflammation has become the most successful new-product launch in the history of the pharmaceutical business, eclipsing the anti-impotence drug Viagra. About 6 million prescriptions for the drug, Celebrex, have been filled in less than six months.

Celebrex, developed by the G.D. Searle unit of Monsanto Co., is the first of a new class of anti-inflammatory drugs designed to be particularly useful in chronic ailments like arthritis. The second drug in the class, developed by Merck & Co. and known as Vioxx, was approved last month, and about 126,000 prescriptions were written for it in its first 30 days on pharmacy shelves.

A slew of similar compounds are under development at other companies. The new drugs have been widely referred to as "super aspirins," a somewhat misleading moniker because it implies that they offer dramatic pain relief. They offer about the same degree of pain relief as older medicines like aspirin, Advil and Aleve, but they appear to be much safer when taken over long periods at high doses.

Many people, particularly people with arthritis who feared taking strong painkillers because they can cause ulcers, digestive bleeding and other serious problems, are finding relief with the new treatments.

"It was like Midas had touched me and I was a pot of gold," said Margaret Burton of Olney, who started taking Celebrex for her arthritis this year. "I just felt like a different woman. It's like the fountain of youth, to put it mildly."

The drugs, though expensive and not without risks of their own, offer a new option to satisfy one of mankind's oldest needs.

Modern pharmacology began in 1893 when a German chemist for Bayer AG, worried about his father's arthritis, tweaked a substance from willow tree bark to produce acetylsalicylic acid, better known as aspirin. The drug suppressed an enzyme known as cyclo-oxygenase, or COX, which plays an important role in pain and inflammation. Many of the painkillers developed in succeeding decades worked the same way.

But in recent years scientists discovered that COX comes in two forms, dubbed COX-1 and COX-2. COX-1 seemed to play an important role in protecting the lining of the stomach and intestines, while COX-2 seemed more directly involved in the pain and inflammation of diseases like arthritis. Older drugs inhibit both enzymes, which partly explains their ability to harm the stomach. Frenzied work began to develop potent, highly specific inhibitors of the COX-2 enzyme. It is these drugs that are now coming to market.

"This is an advance in our basic understanding of molecular chemistry that has made it from the laboratory to the bedside," said Herbert S. B. Baraf, a Washington-area rheumatologist who helped research the new drugs. "It should make a tremendous difference. I really believe that these drugs will become the standard of care" for arthritis.

In large-scale human tests, they have proven safer than older drugs, though the full story won't be known until they have been on the market for years and been taken by tens of millions of people. So far, there seems to be no unexpected elevation in the rate of deaths among people taking Celebrex, the most widely used COX-2 inhibitor. By contrast, aspirin and similar drugs are believed to cause 5,000 to 10,000 deaths a year in the United States, primarily by damaging the intestinal tract.

Many of the 22 million Americans with arthritis already swear by the new treatments. Some of them could not take older drugs because they had suffered stomach damage or were at high risk, and they were consigned to weak arthritis treatment or none at all.

Celebrex "has improved my life incredibly," said Laurie Schuster, 38, of Chicago, who has been battling arthritis since she was 11 and had stomach problems caused by older drugs. "I used to have to take . . . all kinds of medication to ward off an ulcer. You had to work your life around the medication. Now I can just take it and not have to worry."

Laurie Stollery, 60, of Rochester, N.Y., had to give up her beloved golf games when an attack of arthritis left her debilitated. She took older painkillers but had to accompany them with medicines to prevent stomach damage. Taking Celebrex has simplified her regimen, and it works so well she plays golf every Tuesday.

One drawback to the new drugs is cost: $2.50 to $3 per day for treating arthritis, three times the cost of treatment with older painkillers. Moreover, while the risk of stomach damage appears lower than with older drugs, there is believed to be some risk, and the new drugs, like older painkillers, can cause serious kidney problems in a few people.

Finally, the new drugs don't seem to work for everyone. As with older treatments, genetic variations seem to play a role in how people respond.

Still, experts are generally convinced the drugs are safer than the ones they are meant to replace. Particularly in arthritis patients who have had stomach problems taking existing drugs, doctors are racing to write new prescriptions.

The stocks of Merck and Monsanto have not reacted much to the new drugs, in part because investors took their likely success into account long ago. Still, among Wall Street analysts and investors who follow drug stocks, enthusiasm for the products is running high.

Jim Flynn, managing director at ING Baring Furman Selz, said both Celebrex and Vioxx could eventually achieve $5 billion a year in worldwide sales. This would put them in a class with cholesterol-lowering drugs like Lipitor as among the world's most successful pharmaceuticals.

CAPTION: Celebrex was the first of a new class of drugs designed to relieve chronic ailments like arthritis. The second, Vioxx, was approved last month.

CAPTION: Laurie Stollery, 60, of Rochester, N.Y., had to give up golf because of debilitating arthritis. She now takes Celebrex and plays golf every Tuesday.