The annual number of children prescribed anti-psychotic drugs jumped fivefold between 1995 and 2002, to an estimated 2.5 million, the study said. That is an increase from 8.6 out of every 1,000 children in the mid-1990s to nearly 40 out of 1,000.

But more than half of the prescriptions were for attention deficit and other non-psychotic conditions, the researchers said.

The findings are worrisome "because it looks like these medications are being used for large numbers of children in a setting where we don't know if they work," said lead author Dr. William Cooper, a pediatrician at Vanderbilt Children's Hospital.

The increasing use of anti-psychotics since the mid-1990s corresponds with the introduction of costly and heavily marketed medications such as Zyprexa and Risperdal. The packaging information for both says their safety and effectiveness in children have not been established.

Anti-psychotics are intended for use against schizophrenia and other psychotic illnesses.

However, attention deficit disorder is sometimes accompanied by temper outbursts and other disruptive behavior. As a result, some doctors prescribe anti-psychotics to these children to calm them down — a strategy some doctors and parents say works.

The drugs, which typically cost several dollars per pill, are considered safer than older anti-psychotics — at least in adults — but they still can have serious side effects, including weight gain, elevated cholesterol and diabetes.

Anecdotal evidence suggests similar side effects occur in children, but large-scale studies of youngsters are needed, Cooper said.

The researchers analyzed data on youngsters age 13 on average who were involved in annual national health surveys. The surveys involved prescriptions given during 119,752 doctor visits. The researchers used that data to come up with national estimates.

Cooper said some of the increases might reflect repeat prescriptions given to the same child, but he said that is unlikely and noted that his findings echo results from smaller studies.

The study appears in the March-April edition of the journal Ambulatory Pediatrics.

Heavy marketing by drug companies probably contributed to the increase in the use of anti-psychotic drugs among children, said Dr. Daniel Safer, a psychiatrist affiliated with Johns Hopkins University, who called the potential side effects a concern.

Safer said a few of his child patients with behavior problems are on the drugs after they were prescribed by other doctors. Safer said he has let these children continue on the drugs, but at low doses, and he also does periodic tests for high cholesterol or warning signs of diabetes.

Dr. David Fassler, a University of Vermont psychiatry professor, said more research is needed before anti-psychotics should be considered standard treatment for attention deficit disorders in children.

"Given the frequency with which these medications are being used, there's no question that we need additional studies on both safety and efficacy in pediatric populations," Fassler said.