A GENETIC test may help prevent children being overdosed on psycho-stimulant drugs like Ritalin, an Australian expert says.

About 50,000 Australian children are prescribed stimulants to treat attention deficit hyperactivity disorder (ADHD), but problems getting the dose right mean many are initially over-drugged.

Studies in adults with the condition have revealed a gene makes some more sensitive to the medication and prone to the "zombie-like" side effects of overdose.

Overdoses affect children in the same way, making them obsessive, introverted, highly focused and unable to change their attention from one thing to another.

ADHD expert Professor Florence Levy, of the University of NSW's school of psychiatry, said testing to see whether the gene could help get the dose right for young sufferers.

"It has been very hard to pr ...