The first long-term government study of American pre-school children who take Ritalin, the attention deficit disorder (ADD) drug, warns of side effects but also found benefits in youngsters with severe problems.
The drug is not approved for use in children under six and the researchers said those youngsters needed close monitoring.
Pre-schoolers were more likely than older children to develop side effects, experts said.
The research was carried out because of concerns over reports that soaring numbers of pre-schoolers were being prescribed psychiatric drugs, including Ritalin.
The study's message was proceed with caution, said Dr Thomas Insel, director of the National Institute of Mental Health. "We're not talking about fidgety three-year-olds," said Dr Insel, whose agency funded the study.
The research involved children with severe attention deficit hyperactivity disorder with behaviours that included hanging from ceiling fans, jumping off slides or playing with fire.
The researchers say the benefits of low-dose treatment outweigh the risks for these youngsters.
But critics dispute that. "I hope publication of this does not lead to more over-prescribing," said Dr Sidney Wolfe of the watchdog group Public Citizen. "The safety isn't adequately established, the efficacy even less."
About 40 per cent of children developed side effects and about 11 per cent dropped out because of problems including irritability, weight loss, insomnia and slowed growth.
Pre-schoolers on methylphenidate, or generic Ritalin, grew about half an inch less and gained about 2lbs less than expected during the 70-week study.
The study appears in the November edition of the Journal of the American Academy of Child and Adolescent Psychiatry.