pending on drugs to treat children and adolescents for behavior-related disorders rose 77 percent from 2000 to the end of 2003, according to a study of prescription purchases by Medco Health Solutions, a pharmacy benefits management company.

The increase, to $536 a patient a year on average, reflected rising prices as growing numbers of young people used newer and more expensive drugs, said Robert S. Epstein, chief medical officer of Medco. The report is to be released today.

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Sales of the behavioral drugs are growing faster than any other type of medicine taken by children, pulling ahead of the previous leaders, antibiotics and asthma treatments, he said. Most of the drugs were treatments for depression and attention deficit disorder, including prescriptions combining both treatments for the same patient.

Use of attention disorder drugs by children under age 5 rose 49 percent from 2000 to 2003, to half of all children taking any behavior-related medication. Scientists who have studied the trend called for more research on side effects and benefits.

"The benefits and risks of using these drugs in a preschool population should be studied systematically," said Dr. Julie Magno Zito, an associate professor of pharmacy and medicine at the University of Maryland who directs a long-running study of pediatric drugs.

The number of children in the sample of 300,000 taking antidepressants rose 15 percent in the first three months of this year, compared with the first quarter of 2003, Medco said. Last year, 65 percent of all children and adolescents taking behavioral medicines were on antidepressants.

Many of the children were taking both antidepressants and attention disorder drugs, or combinations of other behavioral medicines. Dr. Zito said there had been "a huge growth" in children taking combinations of these drugs although clinical studies of the risks have had little attention.

In March, the federal Food and Drug Administration ordered manufacturers to include warnings of a risk of dangerous side effects, including suicide, on these products. The Medco study did not review prescribing patterns after the F.D.A. order, Dr. Epstein said.

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