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In the past few years, ADHD – attention deficit hyperactivity disorder – has become the go-to diagnosis for kids who can't sit still in school. Today, almost every class includes some kid who's on Ritalin, Adderall or another stimulant. These medications calm them down and improve their focus. But astonishingly, their long-term effects are largely unknown. We've been conducting a vast, uncontrolled experiment on our children, with no idea whether the benefits outweigh the costs.

A startling new study of thousands of Canadian children should make us think hard. It found that overall, these drugs have no long-term benefits whatsoever. If anything, kids on Ritalin had worse outcomes than other kids.

The authors of the study took advantage of a "natural experiment" that began in 1997. That year, Quebec introduced mandatory drug insurance – the only province in the country to do so. When access to drugs expanded, Ritalin prescriptions shot up. Within a decade, children in Quebec were using Ritalin at twice the rate of children in the rest of Canada. By 2007, 44 per cent of Canada's ADHD prescriptions were being written in Quebec.

One of the authors of the study is Mark Stabile, professor of economics and public policy at the University of Toronto. If Ritalin is beneficial, he told me, then long-term school performance – as measured by indicators such as graduation rates and math scores – should improve. But the study found no such improvement. In fact, it found that boys who had taken Ritalin were more likely to drop out of school, and girls were more likely to be unhappy and depressed. "Overall, we find considerable evidence of a decline in both behavioural and educational outcomes following the increase in prescription drug coverage and the corresponding increase in Ritalin use," the authors concluded.

"We went into this thinking we'd see benefits," says Prof. Stabile. "We were surprised."

It's not that stimulants make kids dumber. But when they quiet down and become less disruptive, everybody thinks the problem has been solved. The behavioural problems may have gone away. But the underlying issues go unaddressed, and so these kids may not get the extra help they need with learning and math. As Prof. Stabile puts it, "Ritalin is keeping the problem quiet."

For a small but significant percentage of children, effective treatment for ADHD is a lifesaver. We should never lose sight of this. But clinicians have been shocked by the soaring number of children under treatment. In the United States, an astonishing 11 per cent of all schoolchildren – and nearly 20 per cent of high-school-age boys – have been diagnosed with ADHD, and two-thirds of kids with current diagnoses are being treated. Rates in Canada are significantly lower, but the evidence for overdiagnosis is persuasive. One Canadian study, for example, found that the youngest children in a class are significantly more likely to be diagnosed and treated than the older ones, simply because they're more immature.

Yet it will be extremely hard to turn back the tide. ADHD diagnoses are highly subjective, and ADHD drugs are a gold mine for the industry. These drugs are marketed heavily to doctors (and, in the United States, directly to families). Some parents push for a diagnosis because they think medication will improve their children's grades, and university students use them to pull all-nighters. Worst of all, new diagnostic guidelines have loosened the criteria so much that some doctors are now writing prescriptions for patients in diapers.

We are medicating millions of kids for the malady of childhood. If that isn't malpractice, then what is?