It also showed in dramatic fashion how much more prone people with ADHD are to break the law — four to seven times more likely than others.
The findings suggest that Ritalin, Adderall and other drugs that curb hyperactivity and boost attention remain important beyond the school-age years and that wider use of these medications in older patients might help curb crime.
"There definitely is a perception that it's a disease of childhood and you outgrow your need for medicines," said Dr. William Cooper, a pediatrics and preventive medicine professor at Vanderbilt University in Nashville. "We're beginning to understand that ADHD is a condition for many people that really lasts throughout their life."
He has researched ADHD but had no role in the new study, which was led by Paul Lichtenstein of the Karolinska Institute in Stockholm. The findings were published in Thursday's New England Journal of Medicine.
About 5 per cent of children in the U.S. and other Western countries have attention deficit hyperactivity disorder, which can cause impulsive behaviour and difficulty paying attention. Many youngsters are given medication to help them sit still and focus in school. Some people have symptoms into adulthood.
"It's well known that individuals with ADHD have much higher rates of criminality and drug abuse than people without ADHD," but the effect of treatment on this is not well known, Lichtenstein said.
Using Swedish national registers, researchers studied about 16,000 men and 10,000 women ages 15 and older who had been diagnosed with ADHD. The country has national health care, so information was available on all drugs prescribed.
Court and prison records were used to track convictions from 2006 through 2009 and see whether patients were taking ADHD drugs when their crimes were committed. A patient was considered to have gone off medication after six months or more with no new prescription.
For comparison purposes, researchers matched each ADHD patient with 10 similar people without the disorder from the general population.
— About 37 per cent of men with ADHD were convicted of at least one crime during that four-year period, compared with just 9 per cent of men without ADHD. For women, the crime rates were 15 per cent with ADHD and 2 per cent without it.
— Use of ADHD medicines reduced the likelihood of committing a crime by 32 per cent in men and 41 per cent in women.
The crimes were mostly burglaries or thefts. About 4,000 of more than 23,000 crimes committed were violent. ADHD medication use reduced all types of crime, Lichtenstein said.
Cooper called the results striking. "I was surprised by the magnitude of the effect of the medications and the fact that it was so consistent across all the analyses they did," such as the type of drug being used and the types of crimes committed, he said.
The Swedish Research Council, the U.S. National Institute of Child Health and Human Development, the Wellcome Trust and other agencies paid for the research.
ADHD medicines may help people organize their lives better and reduce impulsive behaviour. They also bring a patient into counselling and health care, said Philip Asherson, a professor at the Institute of Psychiatry at King's College London.
"It's not necessarily just the medication" that is reducing the likelihood of crime, he said.
Still, Asherson said the study should lead to wider use of the drugs: "It firmly establishes the link between ADHD and criminality and establishes that medication has an impact on that criminality."
AP Medical Writer Maria Cheng in London contributed to this report.
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