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.

A 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.

After roughly 11 weeks, children who took Zyprexa gained an average of 18.7 pounds, those on Seroquel gained 13.4 pounds, those on Risperdal gained 11.7 pounds and those on Abilify gained 9.7 pounds.

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