Doctors using broader definitions of attention deficit hyperactivity disorder, or ADHD, risk prescribing "unnecessary and possibly harmful treatment" to their patients, experts have warned.

Prescription rates for ADHD drugs such as Ritalin have soared in the UK in recent years, from 420,000 in 2007 to 657,000 in 2012. The condition, the symptoms of which include a short attention span and restlessness, is believed to affect between two and five per cent of school-age children.

However, in a study published in the British Medical Journal today, researchers from the Bond University in Australia point out that the clinical definition of ADHD has been expanded in recent years and urge doctors to be "conservative" when diagnosing the condition.

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To be diagnosed with ADHD, a child or adult must meet diagnostic criteria outlined in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM), a new edition of which was published last year, or in the World Health Organisation's International Classification of Diseases guidelines.

While acknowledging that part of the rise in diagnosis was down to doctors becoming better at recognising the symptoms of ADHD, the study's authors said that the DSM's definition of the condition had been broadened in more recent editions and that guidelines in the UK, the US and Australia offered no definitions of mild ADHD, as opposed to more severe types.

"The broadening of diagnostic criteria in DSM-5 [the most recent edition] is likely to increase what is already a significant concern about overdiagnosis," the authors conclude. "It risks resulting in a diagnosis of ADHD being regarded with scepticism, to the harm of those with severe problems who unquestionably need sensitive, skilled specialist help and support."

They recommended other countries adopt the "conservative treatment approach" recommended by the UK's National Institute for Health and Care Excellence (NICE).

Commenting on the paper, Professor Eric Taylor of King's College Institute of Psychiatry, said the NHS applied "strict criteria" on ADHD diagnosis.

"Probably too few children here get help," he said. "The increase [in diagnosis] in the UK follows an increased ability in the medical profession to recognise ADHD; but all too many children with severe problems still go untreated."

Professor Philip Asherson, a consultant psychiatrist at the Maudsley Hospital said: "Clinicians need to be aware of labelling normal developmental change, while at the same time being aware that ADHD can be a serious condition that requires treatment. The authors provide a sensible approach to deal with this problem in children."