The use of drugs, such as Ritalin, to control hyperactivity among primary children in New South Wales has nearly doubled in the past 10 years.

Statistics from the state health department show that one in 36 boys in the upper years of primary school now takes medication for the attention deficit hyperactivity disorder. Although the prescription rate for boys is four times higher than that for girls, the use of the drugs by girls is also growing.

The figures suggest that one child in every two classes may be being treated for the disorder. The medication works by increasing dopamine levels in the brain, although some experts are doubtful about the long-term effectiveness of such treatment which acts as a stimulant.

As many as one in 20 children is believed to suffer some degree of ADHD but not all need medication.

The health departent says children aged seven to 11 are most likely to be prescribed the drugs. But prescriptions continue to be used widely among the 12 to 15 age group, before falling to less than 1 per cent among 16 and 17-year-olds. Even among pre-school children, one in every 400 four-year-old boys is given either Ritalin or dexamphetamine.

Prescription drug use varies across New South Wales. In the Hunter River region, north of Sydney, children are nearly twice as likely as the state average to take stimulants, while in the far west the use is much less, apparently because of fewer specialised medical services.

A department spokesman said that under NSW law only child psychiatrists and paediatricians were permitted to prescribe drugs for ADHD. If a doctor wants to give the drugs to a child under four, the case has to be evaluated by an independent panel.