Recent cannabis users may be more likely to experience memory problems or difficulties with cognitive function than those who don’t use the drug, says a study led by McMaster University researcher James MacKillop.

The professor of psychiatry holds the Peter Boris Chair in Addictions Research and he is director of the Michael G. DeGroote Centre for Medicinal Cannabis Research.

Using data on more than a thousand individuals, his research team examined history of cannabis use and whether tetrahydrocannabinol (THC), the psychoactive constituent in cannabis, was present in participants’ urine. These cannabis indicators were then examined in relation to neuropsychological performance, which assessed abilities in the areas of memory, attention, processing speed, executive function and motor skills.

Of those interviewed, 135 people, or 12%, had positive urine screens for THC, which suggested they had recently used cannabis. The study found that those who tested positive for THC scored worse on tests of episodic memory and of mental processing speed. None of the other brain functions showed a difference.

“This has implications for all cannabis users, but especially medical cannabis users who may be consuming daily or multiple times daily for symptom management,” said MacKillop.

“These individuals would likely have THC chronically circulating in their system and, in turn, cognitive consequences. This applies to heavy recreational users who are consuming daily too of course.”

He added that the study also suggest that people in high-stakes professions that require split-second decisions like police and air traffic controllers might be impaired on the job even when they limit cannabis use to their time off.

“Some cognitive consequences are present as long as THC is still detectable in urine,” MacKillop said. “Although the effects were very small, any (reduction) in performance may have significant consequences in a critical situation.”

The study has been published in the Journal of Psychiatry and Neuroscience.