In a series of tests, low and moderate users of the drug outperformed heavy users - people taking two or more grams a week - consistently.

The study adds to a growing body of evidence suggesting that drug abuse can cause long-term problems well after the user has stopped taking the drug.

The study, conducted by researchers at the US National Institute on Drug Abuse (NIDA) and Johns Hopkins University School of Medicine, was published in the Journal of Neuropsychiatry and Clinical Neurosciences.

Long-term effects

Dr Alan Leshner, director of the NIDA, said: "This study adds to the accumulating, and worrisome, evidence that heavy use of cocaine can result in persistent deficits in the skills needed to succeed in school and on the job. Cocaine users are risking their futures.

Researchers identified the part of the brain most affected

"For them, prevention and effective treatment become critical public health priorities."

The tests measured verbal memory, manual dexterity, and other cognitive skills. Those who took the most cocaine performed worst.

Dr Karen Bolla, associate professor of neurology at Johns Hopkins led the research. It complements work she published last December showing that ecstasy use impairs memory.

"These findings underscore the connection between cocaine use and neurobehavioral effects," she said.

"While the intensity (grams per week) of cocaine use was more closely associated with decreased performance than the duration of use, all cocaine users in the study experienced reduced cognitive function."

Comparisons

The team studied 30 people who had used cocaine at least four times a month for at least a year. They gave urine samples when they entered the programme to prove that they had recently taken the drug.

They were compared to 21 people who had no history of drug use excepting nicotine, and who had not had more than four alcoholic drinks in the previous month.

All participants were similar in terms of age, education and intelligence, and none had any history of mental illness.

On the 28th or 29th day after entering the research units, the cocaine users took the tests, and the most significant difference in performance was seen on tasks using the prefrontal cortex part of the brain.

This area is responsible for attention, concentration, planning, and reasoning.

In addition to impaired skills, the heaviest cocaine users showed slower reaction times and poorer attention and concentration.