Cocaine and amphetamine drug users struggle with the residual effects for up to a year after going cold turkey, research suggests.
Cocaine has a profound effect
Previous studies have shown that former cocaine-users show impaired concentration, memory and learning skills up to six months after kicking the habit.
But new research has found that the effects last for up to twice as long.
Scientists from Massachusetts General Hospital in Boston studied 50 sets of twins, in which one had previously used cocaine or amphetamines, and the other had not.
They found that people who had used drugs showed diminished concentration and motor skills in comparison with their sibling a year after quitting.
Researcher Dr Rosemary Toomey, said: "These findings provide evidence of long-term residual effects of stimulant abuse."
Both cocaine and amphetamines disrupt the chemical balance of the brain.
There is some evidence to suggest that both drugs stimulate the release of chemicals which cause the blood vessels in the brain to constrict, reducing oxygen supply and leading to cell death.
John Marsden, of the charity Action on Addiction, told BBC News Online: "These results may have important implications for the treatment of recovering stimulant abusers.
"An important element of the treatment process for cocaine and other stimulant addicts is the use of psychosocial counselling.
"But if their concentration and learning skills are impaired for so long, recovering addicts may be less able to benefit from this type of treatment.
"Treatment would then need to be more specifically targeted to individuals' requirements.
"Clearly much more research needs to be done in this area before treatment implications can be fully assessed.
"However the suggestion is that action to combat stimulant addiction may have to be rethought. "
The research is published in the journal Archives of General Psychiatry.