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Marijuana Use Can Cause Laziness — At Least in Rats

Marijuana Use Can Cause Laziness — At Least in Rats

A new study has found that tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana, makes rats less willing to try a cognitively demanding task.

“Perhaps unsurprisingly, we found that when we gave THC to these rats, they basically became cognitively lazy,” said Mason Silveira, the study’s lead author and a Ph.D. candidate in the department of psychology at the University of British Columbia.

“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 study may throw some light on what has been called amotivational syndrome in humans who smoke marijuana heavily for a long time. While still a point of debate among experts and researchers, the syndrome is characterized by apathy, dullness, lethargy, and impaired judgment, but only affects some heavy users.

For this study, published in the Journal of Psychiatry and Neuroscience, researchers looked at the effects of both THC and cannabidiol (CBD) on rats’ willingness to exert cognitive effort.

They trained 29 rats to perform a behavioral experiment in which the animals had to choose whether they wanted an easy or difficult challenge to earn sugary treats.

At the beginning of each experiment, the rats chose between two levers to signal whether they wanted an easy or hard challenge.

Choosing the easy challenge resulted in a light turning on for one second, which the rats could easily detect and respond to by poking it with their nose, receiving one sugar pellet as a reward. In the more difficult challenge, the light turned on for only 0.2 seconds, rewarding the rat with two sugar pellets if they responded with a nose poke.

Under normal circumstances, most rats preferred the harder challenge to earn a bigger reward, according to the researchers. But when the rats were given THC, the animals switched to the easier option, despite earning a smaller reward.

When the scientists looked at the effect of CBD, an ingredient in marijuana that does not result in a high, researchers found the chemical did not have any effect on rats’ decision-making or attention.

CBD, which is believed to be beneficial in treating pain, epilepsy and even cancer, also didn’t block the negative effects of THC.

“This was surprising, as it had been suggested that high concentrations of CBD could modulate or reduce the negative effects of THC,” said Dr. Catharine Winstanley, senior author of the study and an associate professor in University of British Columbia’s department of psychology. “Unfortunately, that did not appear to be the case.”

Given how essential willingness to exert cognitive effort is for people to achieve success, Winstanley said the findings underscore the importance of realizing the possible effect of cannabis use on affecting the willingness to engage in harder tasks.

The findings highlight a need for more research to determine what THC does to the human brain to alter decision-making, the researchers noted. That could eventually allow scientists to block these effects of THC, allowing those who use medical marijuana to enjoy the possible benefits of cannabis without the less desirable cognitive effects, they said.

Source: University of British Columbia

Marijuana Use Can Cause Laziness — At Least in Rats

Janice Wood

Janice Wood is a long-time writer and editor who began working at a daily newspaper before graduating from college. She has worked at a variety of newspapers, magazines and websites, covering everything from aviation to finance to healthcare.

APA Reference
Wood, J. (2018). Marijuana Use Can Cause Laziness — At Least in Rats. Psych Central. Retrieved on November 23, 2020, from
Scientifically Reviewed
Last updated: 8 Aug 2018 (Originally: 28 Aug 2016)
Last reviewed: By a member of our scientific advisory board on 8 Aug 2018
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