Since it came on the market last year, the drug Crestor has quickly helped millions of people with high cholesterol get their blood lipids down to healthier levels. It is the most powerful statin drug on the market, and many doctors and patients have warmly embraced it.

Not so the consumer advocacy group Public Citizen, which has waged a campaign to have Crestor taken off the market. In a citizen's petition to the Food and Drug Administration, the group said 29 cases of kidney failure or insufficiency that may have been related to Crestor have been reported.

Sidney Wolfe, director of Public Citizen's Health Research Group, argues that Crestor has a safety record uncomfortably similar to that of Baycol, Bayer Corp.'s powerful statin that was taken off the market in 2001 because of related serious side effects. "This is a drug with unique dangers but no real unique benefits," he said.

AstraZeneca Inc., which makes Crestor, disputes Wolfe's conclusions and says the drug is as safe as any other statin. The company's reviews of the 29 reports of "adverse events" showed they had no discernible connection to Crestor, said James Blasetto, AstraZeneca's executive director for strategic development. "The clinical evidence here does not suggest a causal relationship between Crestor and these events," he said.

The distance between the two positions illustrates how difficult -- and important -- the monitoring of approved drugs has become.

A number of researchers, as well as Public Citizen, strongly argued that the arthritis drug Vioxx should have been taken off the market years before its maker, Merck & Co., acted. But with 10,000 prescription drugs available now, the FDA does not have the resources to aggressively investigate all adverse drug reactions. In addition, the agency is always weighing the benefits to many patients against the risk to some patients in deciding how to proceed.

Still, Americans who need medications rely on the FDA to protect them. As the Vioxx furor made clear, many patients, doctors and lawmakers want more information about possible risks, even if they are inconclusive.

Steven Galson, acting director of the FDA Center for Drug Evaluation and Research, said the agency "has been very concerned about Crestor since the day it was approved, and we've been watching it very carefully." He said the agency is "concerned about the same issues with Crestor as Public Citizen."

Galson said the agency is looking for ways to give the public more information about the evolving risk-benefit equation.

"The public wants more information about our products and our processes," he said. "They want to know incrementally what's going on, and I think we've been too stingy in the past about putting the information out there."