PHILADELPHIA — Talk therapy can work as well as antidepressants in severely depressed people and it should be used as a first line of defense, University of Pennsylvania researchers concluded in a study published yesterday.

In the study of 240 patients, researchers found that cognitive therapy, a type of treatment that teaches patients to think more realistically, worked as well as a popular antidepressant for moderate to severe depression.

Patients who got four months of cognitive therapy also had about the same relapse rate a year later as people who took Paxil (paroxetine) the whole time. If people quit taking Paxil after four months, their relapse rate was twice that of therapy patients'.

As a result, the authors from Penn and Vanderbilt University contend, cognitive therapy is cheaper than antidepressants in the long run.

The research adds to a growing body of evidence that this type of therapy works as well as drugs. The study is important, experts said, because of its size and because it tested therapy in moderately and severely depressed patients, a group where previous results have been mixed.

The research, funded by a $4 million grant from the National Institutes of Health, was published in the Archives of General Psychiatry.

In the study, patients on medication got better quicker. At eight weeks, the response rate was 50 percent for Paxil, 43 percent for cognitive therapy and 25 percent for placebo. But by 16 weeks, 58 percent of patients in both treatment groups were feeling better.

Lead researchers Robert DeRubeis, a Penn psychologist, and Steven Hollon at Vanderbilt argued that the American Psychiatric Association should change its treatment guidelines for moderate to severe depression, which currently call for antidepressants as the first-line treatment.

That's unlikely to happen soon, said Dr. Laura Fochtmann, a psychiatrist at Stony Brook University who is the guidelines' medical editor. They were last revised in 2000 — a two-year process that involved analyzing all the available research. "We can't really revise them for every new study that comes out," she said.

In the cognitive approach, patients typically practice thinking about how realistic their ideas really are, said Judith Beck, a psychologist in Bala Cynwyd, Pa. They get "homework" to get out and do things they enjoyed before they got sick.

"They walk out the door with skills they can use to manage their own moods," Hollon said.

In the study, more experienced cognitive therapists got better results, a finding that could be problematic for potential patients. DeRubeis estimates that 10 to 30 percent of therapists practice cognitive therapy.

While many people are wary of antidepressants, some, he and other therapists conceded, prefer the pharmaceutical approach.

"The drugs are much easier and they're much more widely available," said James Herbert, a Drexel University psychologist.