A popular antidepressant plus three months of psychotherapy dramatically helped children with anxiety disorders, the most common psychiatric illnesses in kids, the biggest study of its kind found.

The research also offers comfort to parents worried about putting their child on powerful drugs — therapy alone did a lot of good, too.

Combining the drug sertraline, available as a generic and under the brand name Zoloft, with therapy worked best. But each method alone also had big benefits, said Dr. John Walkup, lead author of the government-funded research. It's estimated that anxiety disorders affect as many as 20 percent of U.S. children and teens.

In many cases, symptoms almost disappeared in children previously so anxious that they wouldn't leave home, sleep alone, or hang out with friends, said Walkup, a Johns Hopkins Hospital psychiatrist.

"What we're saying is we've got three good treatments," he said.

Sertraline is among antidepressants linked with suicidal thoughts and behavior in children with depression.

In this study, only a handful of the more than 200 kids using it had suicide-related thoughts and there were no suicide attempts, Walkup said. Suicidal tendencies are more common in depression than in anxiety, he said.

Zoloft, mostly used to treat adult depression and anxiety, is approved for treating obsessive-compulsive disorder in kids, but not anxiety. Some doctors use it for that, however. And some smaller, less rigorous studies have suggested it and other antidepressants can help.

The new study, paid for by the National Institute of Mental Health, is the largest examining treatment of childhood anxiety disorders, said co-author Dr. John March of Duke University,

Dr. Thomas Insel, the institute's director, said the study provides strong evidence that combined treatment is "the gold standard," but that sertraline or therapy alone can be effective.

Dr. Sharon Hirsch, a University of Chicago psychiatrist not involved in the study, said it echoes benefits she's seen in her own young anxiety patients on both treatments. But she note that the study shows that therapy alone is also good news for parents who don't want to put their children on an antidepressant.

The study, published online in the New England Journal of Medicine, was scheduled for presentation Thursday at an American Academy of Child and Adolescent Psychiatry meeting in Chicago.

Several study authors reported receiving consulting fees or other compensation from drug companies, including antidepressant makers.

The study involved 488 children aged 7 to 17 treated at six centers around the country. They were randomly assigned to one of four 12-week treatments: up to 200 milligrams daily of sertraline; 14 hour-long sessions of psychotherapy alone; both treatments together; or dummy pills.

In the combined treatment group, 81 percent of children were much improved by three months, compared with 60 percent in the therapy-only group, 55 percent in the sertraline-only group, and 24 percent in the placebo group.

Improvement, measured on a psychiatric scale, meant that anxiety had lessened so much that kids could do things they'd refused to do before, such as sleep in their own beds, go to school and socialize.

There was only one serious "adverse event" considered possibly linked to treatment — worsening behavior in a child on drug treatment only.

While many kids have occasional fears or anxiousness, those with full-fledged anxiety disorders are almost paralyzed by these feelings. Three types of disorders were studied: separation anxiety, generalized anxiety and social phobia, Walkup said.

Affected kids may be so worried that something bad will happen to their parents that they repeatedly refuse to go to school. Or they'll be so afraid of thunderstorms that they get chronic stomachaches, even when it's not stormy. Those with social anxiety disorder may just seem shy, but they are so self-conscious that they won't seek out friends or take part in class so their grades suffer, Walkup said.

"These kids were really miserable at the start of the study," and many ended up "really happy," March said.

The therapy used in the study was cognitive behavior therapy, which emphasizes that thoughts can be irrational and cause troubling feelings. It encourages patients to focus on positive thinking that allows them to develop ways of confronting fearful situations.