Parents of children taking medication for attention deficit hyperactivity disorder are nearly 10 times more likely to take the drugs themselves compared with other parents, according to a study of family links to the condition.

The research on prescription claims by 107,000 parents and their children in 2005 supports the observation that ADHD tends to run in families.

When a parent and child both started taking medication, it was the child who started first about 60 per cent of the time, according to the study by prescription benefit manager Medco Health Solutions Inc., which runs employee health plans for companies.

"Heredity has long been suggested as a cause of ADHD, but this is the first study of significant magnitude to confirm the theory," said Dr. Robert Epstein, Medco's chief medical officer.

In homes where both generations started taking the drugs for the first time last year, it was the mother who began treatment, not the father — even though ADHD is two to three times more common in males.

Part of the reason may be that more women are being examined for attention deficit disorder, said Dr. Andrew Adesman, chief of developmental and behavioural pediatrics at Schneider Children's Hospital in New Hyde Park, N.Y.

When ADHD lasts into adulthood, the hyperactivity part often goes away but the inattention persists, experts say.

The study also looked at use of ADHD medications among nearly 17,500 sets of twins.

When one twin was taking the prescription drugs, there was a 25 per cent chance the other twin was also being treated, regardless of their gender. The probability was almost 2.5 times that for non-twin siblings both taking the drugs.

Last week, Health Canada warned that Ritalin and other drugs to treat ADHD have the potential to cause uncommon psychiatric side-effects, including rare cases of agitation and hallucination in children.

With files from the Associated Press