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 predict how kids will respond to treatment," said Prof Levy, who helped establish a government committee to monitor stimulant use.

Stimulant medications for ADHD - methylphenidate, known as Ritalin, and dexamphetamine - work by stimulating dopamine, a chemical which controls several brain functions and relays messages.

International researchers have discovered a gene, called COMT, which controls how much dopamine each person produces.

Adults with one version of the gene produced lower dopamine levels and therefore needed higher doses of the stimulants for the drug to work.

But those with the other version already had high levels and required only very small doses of medication.

"So, essentially the wrong people were being overdosed," said Prof Levy, who has published a review in the latest edition of the Australian and New Zealand Journal of Psychiatry calling for the gene to be studied in children.

"It might just be that we're able to predict how kids will react to the drug in advance, and save them (from these fluctuations).