Using brain imaging, scientists have found that, in pill form, Ritalin - taken by thousands of British children and four million in the United States - occupies more of the neural transporters responsible for the 'high' experienced by addicts than smoked or injected cocaine. The research may alarm parents whose children have been prescribed Ritalin as a solution to Attention Deficit Hyperactive Disorder.

The study was commissioned to understand more about why Ritalin - which has the same pharmacological profile as cocaine - is effective in calming children and helping them concentrate, while cocaine produces an intense 'high' and is powerfully addictive.

In oral form, Ritalin did not induce this intense psychological 'hit'. But Dr Nora Volkow, psychiatrist and imaging expert at Brookhaven National Laboratory, in Upton, New York, who led the study, said that injected into the veins as a liquid rather than taken as a pill, it produced a rush that 'addicts like very much'. Interviewed in last week's Journal of the American Medical Association newsletter, she said: 'They say it's like cocaine.'

Even in pill form, Ritalin blocked far more of the brain transporters that affect mood change and had a greater potency in the brain than cocaine. Researchers were shocked by this finding. A normal dose administered to children blocked 70 per cent of the dopamine transporters. 'The data clearly show the notion that Ritalin is a weak stimulant is completely incorrect,' said Volkow. Cocaine is known to block around 50 per cent of these transporters, leaving a surfeit of dopamine in the system, which is responsible for the hit addicts crave. But now it is known that Ritalin blocks 20 per cent more of these auto-receptors.

'I've been almost obsessed about trying to understand [Ritalin] with imaging,' said Volkow. 'As a psychiatrist I sometimes feel embarrassed [about the lack of knowledge] because this is by far the drug we prescribe most frequently to children.'

However, it was still not clear why a drug that has been administered for more than 40 years was not producing an army of addicted schoolchildren. Volkow and her team concluded that this was due to the much slower process of oral ingestion. It takes around an hour for Ritalin in pill form to raise dopamine levels in the brain. Smoked or injected, cocaine does this in seconds.

Dr Joanna Fowler, who worked with Volkow on the project, said: 'All drugs that are abused by humans release large quantities of dopamine. But dopamine is also necessary for people to be able to pay attention and filter out other distractions.'

But opponents of Ritalin, labelled a 'wonder drug' and a 'chemical cosh', believe it may be addictive and has dangerous side-effects. Moreover, many believe ADHD is a fraudulent title for a non-existent condition once put down to the exuberance of youth. Professor Steve Baldwin, a child psychologist from Teesside University, who died this year in the Selby rail crash, campaigned against Ritalin. He pointed out similarities between the drug and amphetamines as well as cocaine.

Mandy Smith of Banff in Scotland has a son of eight who was prescribed Ritalin for nine months. 'I am astonished the British Government have allowed this drug to be prescribed,' she said. 'It can destroy people's lives. My son was a changed person when he took Ritalin. He was suicidal and depressed.'

Janice Hill, of the Overload Support Network, a charity for parents of children with behavioural problems, said: 'Now we have thousands of children in Scotland taking a drug that is more potent than cocaine. What does it take before the situation is thoroughly investigated?'