A study of more than 25,000 people with ADHD found the number of crimes committed was about a third or more lower in those taking medication, suggesting that encouraging ADHD patients to stay on the pills could cut the risk of crime.

Past international studies show up to two-thirds of young offenders and half the adult prison population screen positively for childhood ADHD, and many may still have symptoms as adults.

British and Swedish researchers who conducted the new study found that patients who went through periods on and off ADHD drugs had a significantly reduced risk of engaging in criminal activity when they were medicated.

"The bottom line is that medication led to a 32 percent reduction in crime rates in men and a 41 percent reduction in crime rates in women," said Seena Fazel, a forensic psychiatrist at Britain's Oxford University who presented the findings at a briefing in London

Paul Lichtenstein of Sweden's Karolinska Institute, who worked with Fazel and colleagues, said the results suggested that encouraging more ADHD sufferers to take medication could help to reduce crime and re-offending rates.

"It's said that roughly 30 to 40 percent of long-serving criminals have ADHD. If their chances of recidivism can be reduced by 30 percent, it would clearly affect total crime numbers in many societies, he said in a statement.

Some 5 percent of school-age children and around half as many adults worldwide have ADHD, a disorder characterized by distractedness and impulsive and sometimes violent behavior.

In the United States, it is one of the most common childhood disorders with an average of 9 percent of children between the ages of five and 17 are diagnosed with it each year.

Previous studies have shown that people with ADHD have on average less education and lower incomes, higher rates of unemployment, divorce and substance abuse, and are more likely to enter a life of crime. But until now health experts were not clear how medication might be affecting the crime risk.

"We've shown that ADHD medication very probably reduces the risk of crime," said Henrik Larsson of the Karolinska Institute. "However ... most medical treatments can have adverse side effects, so risks must be weighed up against benefits."

Ritalin, known generically as methylphenidate, is sold by the Swiss drugmaker Novartis and is widely used in developed countries to help people with ADHD to concentrate better and control impulsiveness. Other ADHD drugs include Johnson & Johnson's Concerta, Shire's Adderall and Vyvanse and Eli Lilly's Strattera.

Philip Asherson, an ADHD expert from Britain's Institute of Psychiatry who was not involved in this research but was speaking at the London briefing, said the findings could point to a cost-effective way to help patients stay out of trouble.

In Britain for example, a month's supply of ADHD medication costs around 300 pounds per patient, he said - a fraction of the cost to society of keeping someone in prison.

Asherson stressed however that decisions about medication should be a personal choice.

The study was published in the New England Journal of Medicine on Wednesday.

(Reporting by Kate Kelland; editing by Stephen Nisbet)

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