Dr. Seth Crockett and his colleagues compared 8,189 cases of inflammatory bowel disease with 21,832 healthy individuals and found the odds of developing such diseases were 1.68 times higher among isotretinoin users. People who had filled four or more prescriptions had 2.67 times the risk. The odds of having ulcerative colitis, a type of bowel disease that causes open sores in the lining of the rectum and colon, were 4.36 times higher among isotretinoin users. (No increased risk was found for Crohn's disease, perhaps the most severe such ailment.)

But only about 5 to 10 people in 100,000 are diagnosed with inflammatory bowel disease each year, Crockett said.

"The absolute risk of getting inflammatory bowel disease is very low," he said. "So if someone has disfiguring acne that was affecting their quality of life, it might be a risk they are willing to take."

Because the data have not yet been published in a peer- reviewed journal, Crockett noted, they must be considered preliminary.

And a similar study, published in July in the American Journal of Gastroenterology by University of Manitoba researchers, found no such association. That study examined a large database in Canada and found that 1.2% of people diagnosed with inflammatory bowel disease had used the drug before diagnosis, compared with 1.1% who had not used isotretinoin.

There is no biological explanation for why isotretinoin might increase the risk of bowel disease, Crockett said, adding: "There are a lot of things that are not understood."

A statement by Roche said its decision to withdraw Accutane reflected market pressures and the cost of lawsuits, not safety concerns.

Accutane costs about $1,200 a month, and many consumers choose the generics, which cost 25% to 50% less. Meanwhile, plaintiffs have won an estimated $33 million in judgments against Roche for bowel disorders, according to an industry publication, Dermatology Times.

The new data are certain to spark renewed opposition to the drug, said Michael S. Brown, an Encino-based lawyer who specializes in personal-injury claims.

The intent of the lawsuits is to drive the medication off the market, he said. "That has been our goal for several years or, in the alternative, to make full disclosure in a way in which the public is properly informed," he said. The isotretinoin patients he represents were not told of a potential increased risk of bowel disease, Brown said.

Taub, the Northwestern dermatologist, says she will include the most recent information on risk in her long discussions with patients who are candidates for the medication. But she worries about a future with nothing to offer people with severe acne.

"If it's financially draining, companies are going to pull out," she said.

"I would hate for anyone to develop ulcerative colitis while treating them for something like acne. But the flip side is that most people who take Accutane have suffered terribly" from acne, she added.