Original cannabis journalism for Canadians

What we know about cannabis tolerance

Tolerance to THC's effects might help explain why heavy cannabis users tend to seek out more potent forms of the drug. (Elaine Thompson / The Associated Press files)

Tolerance to THC's effects might help explain why heavy cannabis users tend to seek out more potent forms of the drug. (Elaine Thompson / The Associated Press files)

Anyone with experience drinking alcohol understands the concept of tolerance: the more frequently you drink, the more booze it takes to get you drunk, or even buzzed. Does the same phenomenon apply to cannabis?

It most certainly does, according to a 2018 scientific literature review published in the journal Neuroscience & Biobehavioral Reviews. The paper summarizes what we know about cannabis tolerance so far.

The authors dug through than 1,000 studies, and identified 36 that measured in some way whether cannabis users develop tolerance to the effects of THC. Breaking the subjects from all those studies down into "regular users" and "non-regular users" of cannabis, the paper explored the evidence on whether those two groups displayed a different tolerance to cannabis.

As you might expect from so many different studies over so many years, results varied, but taken together, the researchers wrote, the best research on cannabis tolerance tells us something important:

"(Cannabis) has less prominent or no effects on a number of behavioural and physiological measures in regular users compared to non-regular users.... the behavioural and physiological effects of cannabis lessen over repeated exposure and often become no longer distinguishable from placebo."

In other words, research suggests cannabis tolerance is objectively measurable, which helps explain why regular users and novices can react differently to the drug. Cognitive functioning (learning and problem-solving ability) seems to be the most likely area where repeated exposure to THC leads to tolerance and a reduced effect, says the paper.

The existence of THC tolerance might also help explain the underlying mechanics behind cannabis use disorder, the clinical term for marijuana addiction or dependency. Since cannabis users can develop tolerance to weed's pleasurable effects, they might start using more cannabis to experience those effects over time. Eventually, that increased use could rise to the level of a diagnosable cannabis use disorder, the authors wrote. 

Some research even suggests cannabis users can reach "full tolerance indicating a complete absence of acute effect," says the review, although the exact biological mechanism through which this occurs isn't fully understood.

That finding raises a rather thorny public policy question: If heavy cannabis users can become so tolerant that they barely feel any effect, is it theoretically possible those users might not have their driving ability impaired after using cannabis, or, at least, that their driving ability could be significantly less impaired than it would be for a novice cannabis user? If so, would that mean anything for cannabis-impaired driving laws?

As with most cannabis-related research questions, a conscientious scientist's response to that theory would inevitably be "more research is needed."

But that kind of research might do more than illuminate nuances in cannabis-impaired driving: a better understanding of how cannabis tolerance works might also help us understand what drives some people to become heavy cannabis users, and how that heavy use affects them.


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