A new study conducted in the Netherlands has shown that heavy cannabis users, who had a history of cocaine use, have increased levels of impulsive behavior.

It had previously been argued that increased impulsivity after cannabis use was only experienced by occasional users, but that heavy users were no longer affected in this way.

The new study, reported in the British Journal of Pharmacology, provides evidence for how regular drug use may trigger impulsive behavior.

In this study, 61 cannabis and cocaine users took both drugs and a placebo in controlled conditions. They then took part in tests that challenged them to reflect before making an action.

“If a person’s tendency to be impulsive increases, they tend to make snap decisions and the error rate increases,” said lead author Dr Janelle van Wel from Maastricht University.

The participants were also studied in situations where they were asked to perform an action, but then told to stop. In this set of tests, people with higher levels of impulsivity make more mistakes and have delayed stopping times. Tests also assessed critical thinking skills, divided attention challenges and aspects of executive function and planning.

Both cannabis and cocaine increased impulsive responding, but in opposite ways.

Under the influence of cannabis, participants were slower, but made more errors. Cocaine administration caused the participants to react more quickly, but if participants had to control their impulses they made more errors.

“This increased impulsivity after drug use could increase the likelihood of developing addiction,” Dr van Wel said.

The findings indicate that long-term users of cocaine and cannabis were more impulsive under the influence of the drugs than when they were given placebos.

“These findings contrast with previous reports that had claimed that these effects after cannabis administration only occurred in occasional users and not in heavy users,” Dr van Wel concluded.

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Bibliographic information: van Wel JHP et al. Single doses of THC and cocaine decrease proficiency of impulse control in heavy cannabis users. British Journal of Pharmacology, published online September 22, 2013; doi: 10.1111/bph.12425