A Canadian researcher on Monday called for an investigation into why children from broken marriages are twice as likely to be prescribed attention-deficit drugs as children whose parents remain together.

University pf Alberta professor Lisa Strohschein reported in the Canadian Medical Association Journal more than 6 percent of 633 children from divorced families were prescribed Ritalin, compared with 3.3 percent of children whose parents stayed together.

"It shows clearly that divorce is a risk factor for kids to be prescribed Ritalin," Strohschein said.

The study of more than 4,700 children started in 1994, while all the families were intact, Strohschein said. They followed the children's progress to see what happened to their families and to see what drugs were prescribed.

Other studies have shown that children of single parents are more likely to get prescribed drugs such as Ritalin. But is the problem caused by being born to a never-married mother, or some other factor?

Ritalin, known generically as methylphenidate, is a psychostimulant drug most commonly prescribed for the treatment of attention-deficit hyperactivity disorder in children.

There is a big debate in much of the developed world over whether it may be over-prescribed - given to children who do not really need it. In March, a University of California, Berkeley study found that the use of drugs to treat ADHD has more than tripled worldwide since 1993.

Strohschein said it is possible that some mental health problems pre-date the divorce, so "it is possible that these kids had these problems before, but are only being identified afterward."