The use of stimulants such as Ritalin to treat hyperactivity is increasingly common in the United States among school-age children. The study discovered a three-fold rise in prescriptions of medical drugs for children under five.

Paedriatricians expressed shock yesterday that the drug-dependent culture was afflicting pre-school children before any conclusive research had been carried out on the effect of psychiatric drugs on brain development. Nor are there clear guidelines on how to tell whether a two-year-old has attention deficit disorder or is just a lively toddler.

The study, published in Jama, the Journal of the American Medical Association, analysed prescriptions to 200,000 toddlers under the national Medicaid and private health systems. Doctors' prescriptions of stimulants (mainly Ritalin) increased from 4.1 to 12.3 per 1,000 pre-school children between 1991 and 1995. The use of anti-depressants increased from 1.4 to 3.2 toddlers per 1,000.

Although psychiatric drugs of all kinds were prescribed to only 1.5% of the toddlers in the study, the trend was sharply upwards and the Jama researchers said they suspected it had continued on its steep trajectory in the five years since the study was conducted.

The recent explosion in the use of "lifestyle" drugs such as the anti-depressant Prozac has generated fears that a significant part of the population is becoming dependent on prescription medicines to solve its problems. Increasingly drugs such as Ritalin have been used to address the problems of "difficult" children.

Nearly 3% of all Americans between the ages of five and 18 are on Ritalin - almost 3m children. The Jama study confirms evidence that surfaced in a smaller survey in Michigan in 1998, that children were being put on medication for supposed psychiatric problems even before they were old enough to go to school.

US experts have suggested that the growing trend towards treating "problem" children with drugs may be a consequence of a health system in which both wholly private and subsidised systems are reluctant to pay for counselling or other non-drug treatments, which are harder to quantify. Paediatricians also point to growing pressure from parents and schools to diagnose troublesome children as having medical problems such as attention deficit disorder.

Steven Hyman, director of the US national institute of mental health, told the New York Times he was "more than shocked" by the findings, adding that the use of psychiatric drugs in young children was "an area of enormous concern".

Reasons given by American doctors for prescribing these medicines include pain relief (analgesics and sedatives/hypnotics), anxiety associated with medical or dental procedures (hydroxyine), bed-wetting in children over six years old (tricyclic antidepressants), and attention deficit hyperactivity disorder in children of three years and older, for which they are given a variety of drugs. There are concerns that some drugs may affect children's growth.

Few of the psychiatric drugs mentioned in the Jama study are approved for treating children of preschool age, and packets of methylphenidate (the generic name for Ritalin) carry warnings against prescribing the drug to children under six.

Most psychiatric drugs work by neutralising or enhancing chemicals in the brain, but animal studies suggest that those chemicals play a critical role in the growth of nerve cells in the brain.

Joseph Coyle, chairman of psychiatry at Harvard Medical School, said: "These interventions are occurring at a critical time in brain development and we don't know what the consequences are."

He argued that the normal behaviour of many two- or three-year-olds is hard to distinguish from hyperactivity.

Julie Zito, an associate professor of pharmacy and medicine at the University of Maryland and the study's lead author, said: "It is not really clear that children this young could meet the diagnostic criteria for either attention deficit hyperactivity disorder or depression."