The number of elementary school students taking medication for attention deficit hyperactivity disorder has climbed steadily in British Columbia to reach a nearly 20-year high despite questions about the long-term effects of the drugs and compelling evidence that some children are being diagnosed inaccurately.

A letter posted this week by the University of B.C.’s Therapeutics Initiative stated that 4.1 per cent of children between the ages of six and 12 are receiving stimulant drug treatments for the disorder, up from 2.4 per cent in 2000.

The letter describes the increase as “unexplained and concerning,” and says it remains unknown whether the benefits of such treatment for ADHD in children outweigh the harms.

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Alan Cassels, communications director at the Therapeutics Initiative, said the drugs have not been studied thoroughly enough to know if there are long-term health effects on developing brains.

“We don’t know what the long-term health impacts of these drugs are,” he said. “We can’t conclude they’re negative – there may be no long-term health impacts – but there also could be harm.”

ADHD is a mental illness commonly characterized by an inability to focus or sit still. According to the Canadian Mental Health Association, it affects boys more often than it does girls and appears to run in families. The CMHA estimates that it affects about 5 per cent of school-age children.

The letter serves as an update to a 2012 study by the same researchers that found children born in the latter months of the year are more likely to be diagnosed with ADHD than their peers born earlier in the year. (Boys were 41 per cent more likely and girls 77 per cent more likely to be prescribed a stimulant medication if they were born between September and December than if they were born in January.)

The study suggested that poor and disruptive behaviour among the youngest children in a classroom might be driving rates of ADHD diagnosis and treatment, and that teachers, parents and physicians are medicalizing a social rather than medical problem.

The study has since been replicated in other countries including the United States, Iceland, Denmark and Germany, and those studies have yielded similar results. The U.S. study found that children born just after the school enrolment cutoff date had a significantly lower incidence of ADHD diagnosis and treatment than those born before, and estimated that “roughly 20 per cent of the 2.5 million children (in the United States) who use ADHD stimulants may have been misdiagnosed.”

“Despite the fact that there’s international attention to this, and concerns about long-term use of ADHD drugs, the prescribing continues to mount,” Mr. Cassels said.

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A 2008 Therapeutics Initiative paper on using central nervous system stimulants to treat ADHD in children concluded that while the drugs improved teacher and parent ratings of hyperactive and impulsive disruptive behaviour, they did not improve children’s ratings of anxiety or measures of academic achievement.

Mr. Cassels emphasized that while some children certainly benefit from stimulant drug treatment, its use and effects must be studied further.

“I would not say there is no place for these drugs,” he said, “but when you have 4 per cent of children between six and 12 being diagnosed with [ADHD], it does raise many questions about whether we’re medicating a social rather than a medical condition.”

The paper noted that children are particularly vulnerable to the harms of long-term drug therapies and said that there should be a higher level of evidence of effectiveness to justify their use.