Some widely prescribed drugs for depression provide relief in extreme cases but are no more effective than placebo pills for most patients, according to a new analysis.

The findings could help settle a longstanding debate about antidepressants. While the study does not imply that the drugs are worthless for anyone with moderate to serious depression -- many such people do seem to benefit -- it does provide one likely explanation for the sharp disagreement among experts about the drugs' overall effectiveness.

Taken together, previous studies have painted a confusing picture. On one hand, industry-supported trials have generally found the drugs sharply reduce symptoms. On the other, many studies that were not initially published, or were buried, showed no significant benefits compared with placebos.

The new report, appearing in The Journal of the American Medical Association , reviews data from previous trials on two types of drugs and finds that their effectiveness varies according to the severity of the depression being treated.

"I think the study could dampen enthusiasm for anti -depressant medications a bit, and that may be a good thing," said Erick H. Turner, a psychiatrist at Oregon Health and Science University. "People's expectations for the drugs won't be so high, and doctors won't be surprised if they're not curing every patient they see with medications."