Preschool children taking stimulants such as Novartis AG's Ritalin for attention deficit hyperactivity disorder should get low doses and be monitored closely, researchers said today.

Children taking the drugs with the active ingredient methylphenidate had slower growth rates than those not taking them and one of 10 dropped out of the study because of "intolerable" side effects, studies in the November issue of the Journal of the American Academy of Child and Adolescent Psychiatry showed. The study lasted more than a year and enrolled 303 children aged 3 to 5 years old.

"Preschoolers may benefit from low doses of medication when it is closely monitored, but the positive effects are less evident and side effects are somewhat greater than previous reports in older children," said Thomas Insel, director of the National Institute of Mental Health, which paid for the study.

Earlier studies of preschoolers had suggested they might suffer more side effects of the drugs, such as sleep and appetite loss, than older children, researchers said in one of five studies appearing in the journal today. The drugs bear "black box" warnings on their labels, the U.S. Food and Drug Administration's strongest caution, because of heart risks.

Low Dosage Key

Keeping doses lower than the 15 milligrams to 50 mg usually given to school age children is key, said study leader Laurence Greenhill, a New York State Psychiatric Institute researcher.

"The best dose to reduce ADHD symptoms varied substantially among the children, but the average across the whole group was as low as 14 milligrams per day," he said in the statement.

Preschool children who took the drugs grew about half an inch less in height and weighed about 3 pounds less than expected based on average growth rates, the researchers said. That mirrored results from a 1999 government study on older, school-aged children.

About 11 percent of the children dropped out of the study because of excessive weight loss and other side effects such as insomnia and "skin-picking behaviors." Changes in blood pressure or pulse were "minimal," the researchers said.

Improvement Seen

Children who took the ADHD drugs had greater reductions in attention problems and hyperactivity than those taking a dummy drug, or placebo, said Mark Riddle, director of the division of child and adolescent psychiatry at Johns Hopkins University School of Medicine in Baltimore. The results showing improvement were based on reports from parents and caregivers.