A group of rats high on THC, the main psychoactive ingredient in marijuana, chose to perform the easiest possible task to get a reward, according to research done at the University of British Columbia.

In a test case of 29 rats, the majority preferred a harder challenge in order to gain a larger batch of sugary treats.

But after being given THC, they chose an easier challenge, even though it gave them a smaller reward.

"Their ability to do the difficult challenge was unaffected by THC. The rats could still do the task — they just didn't want to," said Mason Silveira, the study's lead author and a PhD candidate in UBC's department of psychology.

The study, published today in the Journal of Psychiatry and Neuroscience, may seem like a case of science confirming stereotypes.

But Silveira says it shows the need for more research on how different elements of marijuana can have different effects.

CBD vs. THC

"Research like this goes towards helping understand how in the future we can maximize cannabis so it has therapeutic potential, but these somewhat undesirable side effects can be attended to," he said.

He noted that in the study, the rats were also given cannabidiol (CBD), a cannabinoid in marijuana linked to pain relief, but their behaviour didn't change.

"There's not a lot of study that shows how the ratio of THC to CBD affects your cognition.

It's important to see not just how THC is interacting by itself, but how it might interact with other things found in the plant," said Silveira.

"In the future you can try to. ... develop cannabinoids that block these effects on cognition but still help people with the reasons why they use it for pain relief."

However, Silveira emphasized the preliminary nature of the research.

"Anecdotal evidence is good, you have a friend of a friend who's lazy when they're high, but it's good to have these models in place," he said.

"The brain circuitry [in rats] is similar, so it suggests a similar thing might happen in human, but you really need those human trials."