CHICAGO (Reuters) - Up to a third of children and adolescents who took common antipsychotic drugs for the first time became overweight or obese in as little as 11 weeks, raising their risk for diabetes and heart disease, U.S. researchers said on Tuesday.

Children sit on a pool side during a summer camp for overweight children in Seoul July 28, 2006. REUTERS/Kim Kyung-Hoon

They said doctors who prescribe antipsychotics to children should carefully consider the benefits against the risks, and keep close watch on children taking the drugs.

“These data confirm prior findings that children and adolescents are highly vulnerable to antipsychotic medication,” Dr. Christopher Varley and Dr. Jon McClellan of the Seattle Children’s Hospital wrote in a commentary in the Journal of the American Medical Association.

“These results challenge the widespread use of atypical antipsychotic medications in youth,” they wrote.

Dr. Monica Michell, a child psychiatrist at Lenox Hill Hospital in New York who was not involved with the study, said it “cautions us and underscores the long-term potential harm that these medicines can do.”

“For children who are not psychotic or bipolar, these medicines should be a last resort,” she said by email.

The study looked at four of the most common antipsychotic drugs used in children -- Johnson & Johnson’s Risperdal or risperidone, Eli Lilly’s Zyprexa or olanzapine, Bristol-Myers Squibb’s Abilify or aripiprazole and AstraZeneca’s Seroquel or quetiapine.

A team led by Dr. Christoph Correll of Zucker Hillside Hospital and the Feinstein Institute for Medical Research in New York studied 272 children and teens aged 4 to 19 with bipolar disorder, schizophrenia and disruptive or aggressive behavior spectrum disorders.

After roughly 11 weeks, those who took Zyprexa gained an average of 18.7 pounds (8.5 kg), those on Seroquel gained 13.4 pounds (6.1 kg), those on Risperdal gained 11.7 pounds (3.5 kg) and those on Abilify gained 9.7 pounds (4.4 kg).

Altogether, 10 percent to 36 percent of patients became overweight or obese within 11 weeks.

“The weight gain is dramatic, rapid and pervasive,” Correll said in a telephone interview.

Even so, he said, not all of the drugs performed the same.

Children who took Zyprexa had the most dramatic weight gain and the biggest changes in metabolic factors such as blood sugar, cholesterol, and triglycerides, which can cause heart problems and diabetes.

Abilify, a drug that is usually not linked with weight gain in adults, did cause children to gain weight, but did not raise cholesterol or blood sugar levels.

Correll hopes to conduct more research to determine what caused these differences.

“We will look at genetics and look at blood samples to see what changed early on that predicted weight gain,” he said.

Currently, only two atypical antipsychotics are approved for youth -- Risperdal and Abilify. But in June, a Food and Drug Administration panel of experts backed wider use of Zyprexa, Seroquel and Pfizer’s Geodon for children and teens.

At the time, many panel members expressed concern about rising sales of the drugs to young people and the lack of long-term safety studies.

They were especially worried about the chance the drugs might be misused to treat other conditions such as attention deficit hyperactivity disorder.

The FDA has yet to act on the panel’s endorsement.

Last November, a panel of outside experts called on the FDA and other U.S. health agencies to study the long-term effects of prescribing antipsychotic drugs to children.

Zyprexa is Lilly’s top-selling drug with sales of $4.7 billion last year. Seroquel is the second-best seller for AstraZeneca with 2008 sales of $4.5 billion.