The finding could offer evidence that some people are biologically more prone to drug abuse.

This could help doctors target individuals at risk and offer them preventive treatment, doctors have said.

However, there were other factors at work in drug abuse and these would also have to be considered, they said.

Hyperactive drug response

The researchers examined how a group of me reacted to methylphenidate - a mild stimulant used in the treatment of attention deficit hyperactivity disorder and commonly known by one of its trade names, Ritalin.

The men tried a drug used to treat hyperactive children

Dr Alan Leshner, director of the US National Institute of Drug Abuse, said: "This finding supports the theory that brain chemistry may predispose some people to becoming drug abusers.

"Understanding these biological issues will help us learn why some people are particularly vulnerable to abusing drugs and provides new potential targets for both treatment and prevention efforts."

The researchers, led by Dr Nora Volkow of Brookhaven National Laboratory in Upton, New York, monitored brain scans of 23 men after they took the drug.

When asked to describe the sensation of the drug. 12 of the men said it was pleasant and improved their mood, nine said they did not like the feeling at all while two felt neutral.

Pleasure principle

But those who enjoyed the drug had lower levels of chemical doorways known as dopamine D2 receptors in their brain.

They are used to allow dopamine, a message-carrying chemical, into brain cells.

Scientists have been examining the role of dopamine in brain function since the 1970s and believe it is central to many essential tasks.

It thought to be responsible for producing feelings of bliss and is known as the pleasure chemical.

Among the diseases and conditions linked to dopamine imbalance are schizophrenia, Parkinson's disease and drug abuse.

Complex workings

However, the authors of the new study say the overall process of drug abuse is likely to be more complex than a chemical imbalance.

"The fact that the people in this current study who liked methylphenidate also had low levels of these receptors and yet were not stimulant abusers indicates that other factors besides low D2 levels are also necessary to create a real vulnerability to drug abuse," Dr Volkow said.