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Long-term cannabis users are also more impulsive, says Dutch study

Cannabis also appears to promote more impulsive behaviour in people who regularly use it, not only in occasional users as had been assumed until now. Researchers at Maastricht University demonstrated the same effect with cocaine, in a study which was recently published in The British Journal of Pharmacology. Previously it was assumed that regular users become resistant to these and other side effects of cannabis over time. Increased impulsivity after drug use can increase the risk of addiction.

In the study, under the leadership of PhD candidate Janelle van Wel, 61 regular cannabis users who also occasionally use cocaine were given both drugs (cannabis and cocaine) or a placebo. Subsequently they were asked to perform a number of tasks to measure their reflective and motor impulsivity and psychomotor functions. The subjects were asked to perform an action and then to stop. ‘People who exhibit more impulsive behaviour have a tendency to take hasty decisions, leading them to make more mistakes’, according to Van Wel. Furthermore, they were assessed on critical thinking, dividing their attention and decision-making during the planning and execution of a task.

Long-term cannabis users were more impulsive under the influence of both drugs than subjects who had been given a placebo. Moreover, impulsivity is not the same as speed. After cannabis use the subjects reacted more slowly but made more mistakes, which indicates a decrease in their reflective abilities. After administration of cocaine the subjects reacted more quickly, but when they had to control themselves abruptly (had to control their impulses), they made more mistakes. ‘Earlier animal studies showed that animals who were chronically administered cannabis were less sensitive to the effects of cocaine. However, our research shows that cocaine improves psychomotor function and accelerates reactions, even in heavy cannabis users.’

According to Van Wel, increased impulsivity also increases the likelihood of developing an addiction. ‘A characteristic of drug addiction is a disturbed relationship between the frontal cortex, where we take decisions, and the limbic system that regulates emotional reactions and memory. Our results imply that cannabis diminishes the degree of control that the frontal cortex exerts on someone’s behaviour, while cocaine increases the impulsive reaction of the limbic system. Both options can be explanations for the increase in impulsivity that we saw in our study.’