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There's a running gag in Floater, Calvin Trillin's 1980 comic novel about a newsmagazine that sounds a lot like TIME, in which the medicine writer comes down with the symptoms of whatever disease he's writing about that week. I was reminded of that hapless writer when I read about a new study out of University College London that found that people who use the Web to get information about their chronic diseases often wind up in worse shape than before they logged on.

Could the medical information on the Internet actually be making people ill?

Not exactly, says Dr. Elizabeth Murray of the college's department of primary care and population sciences. She and her colleagues reviewed 28 studies involving 4,042 patients, all experiencing an ongoing illness such as asthma, cancer or diabetes. Consulting Web-based health programs and support groups increased these patients' knowledge and feelings of social support. But it didn't lead to behavior changes, and it had a striking — and surprising — negative effect on outcomes.

One reason for this, Murray speculates, is that patients get so steeped in information that they make their own treatment decisions — ignoring their doctor's advice. Another possibility is that they discover that the unfavorable consequences of their behavior may be a long way off and far from a statistical certainty. "They might appreciate that there's a risk in having a stroke," says Murray, "but not take their meds, knowing there's a possibility that they may not have one."

As a doctor, I am concerned about this. I'm a great believer in patients being well informed, and I'm encouraged by reports like the Pew Internet and the American Life Project that found some 93 million Americans use the Web to research health topics. But there's a difference between having well-developed knowledge about a medical issue and just having loads of information — especially if much of that information is wrong. One study of Web searches for the term vaccinationfound that 43% led to sites advising people not to get vaccines. That's not what I'd call sound advice.

There's plenty of good medical information on the Internet, but you have to look carefully at the source. Who runs the website? How often is it updated? How well is it vetted? The websites sponsored by the National Institutes of Health are generally excellent, as are many sites produced by universities and hospitals. Patient organizations can be wonderful sources of support, but they can be repositories of rumor and misinformation too.

Your health is a partnership between you and your physician. It's fine to use the Internet to understand your health better, but it should never replace your doctor's counsel.

Sanjay Gupta is a neurosurgeon and CNN medical correspondent

• Sanjay Gupta