A spate of overdoses of Ritalin among children and teenagers in Canada’s capital, Ottawa, has renewed calls for curbs on the drug which is used to treat attention deficit hyperactivity disorder.

Critics say that the drug is too easily available in schools and at home.

Between 1993 and 1997 the number of Canadian youngsters taking Ritalin more than trebled to 322,000. The number of prescriptions has increased from 138,000 in 1990 to 652,000 in 1997. Many children have to take two doses a day, which means that a pill is included in the school lunch pack.

Thirty-nine of the 84 cases referred to the Children’s Hospital of Eastern Ontario in 1997 involved children under 13.

Jill Courtemanche, a consultant at the hospital, said: “In most cases these are accidental overdoses; they might be doubling doses or the result of younger kids getting into an older sibling’s things.”

She added that most of the 55 incidents involving teenagers resulted from “experimentation or a deliberate intent to have a high”.

Ritalin, which is classified alongside cocaine as a “controlled substance”, is a stimulant prescribed for children with ADHD because it modifies brain chemicals that affect concentration.

Dr Declan Quinn of Saskatoon’s Royal University, who is conducting studies on a non-narcotic form of Ritalin, says that it causes a “high” comparable to that of heroin when taken in large doses or mixed with other drugs.

Dr Norman Hoffman of McGill University estimates that 2,000 students use it. He said: “It’s becoming a major street drug at McGill.” In 1995 an Ontario teenager was arrested for selling stolen Ritalin.

The explosion in the use of Ritalin in Canada (mirrored in the United States where the numbers of students using it have risen from 900,000 in 1990 to more than 2.5 million today) is reflected in the growth of children diagnosed as having ADHD.

North American practitioners are much more likely to diagnose ADHD than their British counterparts: one in 20 compared with one in 200 in Britain.

Critics such as Vancouver psychologist Thomas Millar, author of The Myth of Attention Deficit Disorder, have argued that in the vast majority of cases ADHD is a misdiagnosis. Too many children who fidget, are talkative, inattentive or easily distracted are said to suffer from ADHD.

Dr Michael Ferri, an Ontario psychologist, said: “A growing number of parents want a biological disorder for their children because it is more palatable than acknowledging that they may not be being reared properly.”