“There are too many children getting on too many of these drugs too soon,” Dr. Mark Olfson, professor of clinical psychiatry and lead researcher in the government-financed study, said.

Image Kyle Warren at 6 years old. At 18 months, Kyle started taking a daily antipsychotic drug on the orders of a pediatrician trying to quell the boy’s severe temper tantrums. Credit... Chris Bickford for The New York Times

Such radical treatments are indeed needed, some doctors and experts say, to help young children with severe problems stay safe and in school or day care. In 2006, the F.D.A. did approve treating children as young as 5 with Risperdal if they had autistic disorder and aggressive behavior, self-injury tendencies, tantrums or severe mood swings. Two other drugs, Seroquel from AstraZeneca and Abilify from Bristol-Myers Squibb, are permitted for youths 10 or older with bipolar disorder.

But many doctors say prescribing them for younger and younger children may pose grave risks to development of both their fast-growing brains and their bodies. Doctors can legally prescribe them for off-label use, including in preschoolers, even though research has not shown them to be safe or effective for children. Boys are far more likely to be medicated than girls.

Dr. Ben Vitiello, chief of child and adolescent treatment and preventive research at the National Institute of Mental Health, says conditions in young children are extremely difficult to diagnose properly because of their emotional variability. “This is a recent phenomenon, in large part driven by the misperception that these agents are safe and well tolerated,” he said.

Even the most reluctant prescribers encounter a marketing juggernaut that has made antipsychotics the nation’s top-selling class of drugs by revenue, $14.6 billion last year, with prominent promotions aimed at treating children. In the waiting room of Kyle’s original child psychiatrist, children played with Legos stamped with the word Risperdal, made by Johnson & Johnson. It has since lost its patent on the drug and stopped handing out the toys.

Greg Panico, a company spokesman, said the Legos were not intended for children to play with — only as a promotional item.

Cheaper to Medicate

Dr. Lawrence L. Greenhill, president of the American Academy of Child and Adolescent Psychiatry, concerned about the lack of research, has recommended a national registry to track preschoolers on antipsychotic drugs for the next 10 years. “Psychotherapy is the key to the treatment of preschool children with severe mental disorders, and antipsychotics are adjunctive therapy — not the other way around,” he said.