Use of prescription stimulants has risen nearly 30 per cent in the past five years, a new study says, but girls are less likely to be taking the drugs than boys, meaning some cases of attention-deficit hyperactivity disorder may be going undiagnosed.

The study, published on Wednesday by the Ontario Drug Policy Research Network, shows prescriptions for stimulants such as Ritalin, Concerta and Adderall are up in all age groups across Ontario. In 2013, 4.7 out of 1,000 people were taking prescription stimulants in Ontario. By 2017, that had risen to six for every 1,000 Ontario residents. Stimulants such as Ritalin and Adderall are approved to treat ADHD and narcolepsy in Canada.

The study found that males are much more likely to receive a prescription for stimulants than females, but the differences are most pronounced among children and teens. For instance, about 5 per cent of boys aged 13 to 18 received a prescription for a stimulant in 2017, the study found, compared with just 2.4 per cent for girls in the same age group.

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The big gap between boys and girls suggests that some cases of ADHD are going undiagnosed, said Doron Almagor, a child and adolescent psychiatrist and chair of the Canadian ADHD Resource Alliance. He said that boys tend to display symptoms of hyperactivity much more frequently than girls and that this sort of behaviour is very quickly noticed by teachers. For girls, ADHD can manifest through symptoms such as an inability to focus and depression, which means the condition is sometimes not recognized for what it is, he said.

“It’s not as visible,” Dr. Almagor said.

Nearly half of the stimulant prescriptions went to people 18 and under, which the researchers said is expected, given that ADHD typically affects young people the most. But the study found the rate of prescriptions going to adults rose significantly in the past five years, which could indicate greater recognition of and treatment for adult ADHD, said Diana Martins, lead author and research program manager at the Ontario Drug Policy Research Network. The drugs may also be increasingly used to treat other conditions such as bipolar disorder or depression, Ms. Martins said.

Researchers attempted to estimate levels of potential inappropriate prescribing of stimulants by measuring early refills of a prior prescription that came from a different doctor and pharmacy. In 2017, less than 1,000 prescriptions were inappropriate – representing 0.12 per cent of all stimulant prescriptions that year. It was a decrease from 1,319 inappropriate prescriptions in 2013.

Peter Selby, a clinician scientist in the addictions division and director of medical education at Toronto’s Centre for Addiction and Mental Health, said the new study raises more questions than it answers. For instance, while the data show more adults are taking prescription stimulants, that doesn’t mean the drugs are being inappropriately prescribed. At the same time, the findings don’t give any insight into how well the drugs are working for those who take them. He said there is a need to better understand how and why people are being prescribed these drugs and whether they are leading to better health outcomes.

“We have no way of knowing people are on the right medication or not,” Dr. Selby said.