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Researchers from the University of British Columbia have explicit sex scenes in mainstream moviesdiscovered some truth in the theory that marijuana makes you lazy -- at least if you're a rat.

In a study published Tuesday in the Journal of Psychiatry and Neurosciene, they found that THC, the main psychoactive ingredient in marijuana, made the creatures less likely to try cognitively demanding tasks.

They used 29 rats for the experiment and tested both the effect of THC and cannabidiol (CBD) on their willingness to exert cognitive effort.

First the rats had to choose between a difficult or an easier challenge, with a larger sugary reward for completing the tougher task.

The easier job involved a light turning on for one second, which the rats could respond to by touching with their nose. For that they received one sugar pellet. The harder task meant the light only turned on for 0.2 seconds, but there were two sugar pellets at stake.

Under normal circumstances most rats preferred the harder task for the bigger reward. After a dose of THC, though, they opted for an easier task for less reward. Because why bother?

When researchers administered CBD, a non-psychoactive ingredient that doesn't result in a high but is thought to have medicinal benefits, they found it had no affect on the rats' decision-making or attention span. They also discovered that CBD didn't block the negative effects of THC.

“Perhaps unsurprisingly, we found that when we gave THC to these rats, they basically became cognitively lazy,” Mason Silveira, the study’s lead author said.

“What’s interesting, however, is that their ability to do the difficult challenge was unaffected by THC. The rats could still do the task— they just didn’t want to.”

The fact that CBD, which can be beneficial in treating pain, epilepsy and cancer, didn't block the effects of THC is significant, senior author of the study Catharine Winstanley said.

“This was surprising, as it had been suggested that high concentrations of CBD could modulate or reduce the negative effects of THC,” she remarked. “Unfortunately, that did not appear to be the case.”

Silveira believes that the results call for further research into the impact of THC on the human brain with the aim of eventually working out how to block its effects. That could allow medical marijuana users to enjoy the benefits without suffering the cognitive effects.