The Troubling Long-Term Effects of Youthful Cannabis Use

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Older adults are using cannabis in increasing numbers for recreational and/or medical purposes and we don't fully understand what its impact may be, because most studies examining cognitive function and cannabis do not include older adults.

A pilot study in 2021 of only 28 adults 60 years and older reported that non-users scored significantly higher than long-term users (at least 19 years) in executive functioning. Short-term users (7 years or less) scored significantly higher than long-term users in both executive functions and processing speed. The researchers concluded short-term use is relatively benign in older adults, while longer-duration use is associated with poorer cognitive functioning.¹ Interesting, but too small a study to provide reliable data.

But hold the presses for breaking news from the March issue of the American Journal of Psychiatry.² The long-term Dunedin study has released important new findings on the cognitive impact of long-term cannabis use in midlife. The original study of more than 1,000 people born in 1972-73 in Dunedin, New Zealand found reduced IQ scores at age 38 in individuals who started using cannabis as young teens. The Dunedin cohort has now reached 45 years old and detailed studies of their cognitive abilities have been repeated, this time with accompanying MRIs of the part of the brain responsible for short-term memory and learning—the hippocampus.²

This new study compared non-users, midlife recreational users, cannabis quitters, and long-term tobacco and alcohol users with long-term cannabis users, defined as using cannabis weekly or more for the past year or meeting research criteria for cannabis dependence at age 45 and also having used cannabis weekly or more at one or more earlier assessments. Of these long-term cannabis users, 31 percent used cannabis before age 18, 89 percent used regularly (four or more days per week) at one or more earlier assessments, and 72 percent met the criteria for cannabis dependence at one or more earlier assessments.

Midlife recreational cannabis users were defined as using cannabis between 6 and 51 days per year at ages 32, 38, or 45. Cannabis quitters did not use cannabis at age 45 but previously either were diagnosed with cannabis dependence or used regularly (four or more days per week).

Researchers summarized the following six findings of note:

1. Long-term cannabis users exhibited IQ decline and poorer learning and processing speed in mid-life, relative to their childhood IQ. People who knew them well described them as having memory and attention problems. Long-term cannabis users had average IQs as children (mean 99.3) but below-average IQs as adults (mean 93.8). Their mean 5.5-point childhood-to-adulthood IQ decline was significantly larger than that observed among lifelong cannabis non-users (mean 0.70).
2. Long-term cannabis users showed significantly larger IQ decline, poorer learning and memory, and poorer processing speed than long-term tobacco or alcohol users.
3. Cognitive functioning among mid-life recreational cannabis users was similar to matched controls. This suggests that infrequent, non-problematic recreational cannabis use in mid-life is unlikely to compromise cognitive functioning.
4. Cannabis quitters showed subtle cognitive deficits that may explain inconsistent findings among studies on the benefits of cessation.
5. Long-term cannabis users showed smaller hippocampus volume compared with non-users.
6. Individuals who used cannabis more persistently had smaller hippocampal volumes than less persistent users, which means the loss of hippocampal volume is dose-dependent.

This important extension of the Dunedin study is likely to be good news for more moderate cannabis users and not so good for more regular users. Personal communication with the article’s lead author, Madeline Meier, confirmed that findings among long-term cannabis users were concentrated among those who began use as young adolescents, consistent with her previous findings. In other words, the early onset of cannabis use may be the primary culprit—an adolescent indiscretion that has a lasting impact on a person’s life.

The loss of hippocampal volume is interesting, but its significance still needs clarification. The hippocampus contains large numbers of cannabinoid receptors (CB1) and THC has been shown to reduce both the number of receptors and hippocampal volume. But the loss of volume is only one, rather gross, method of measuring cannabis’s impact. THC also causes microscopic changes in nerve cells, as well as impacts on white matter connectivity between different parts of the brain. Meier’s group is now analyzing images of other brain areas known to have heavy concentrations of CB1 receptors.

This new data is not a true “smoking gun,” but it is another arrow in the quiver for those studying the potential negative side effects of cannabis on cognition. Most importantly, cognitive childhood-to-adulthood changes such as those observed in the Dunedin cohort have been shown to predict steeper cognitive decline from ages 70 to 82. Such deficits are comparable to mid-life cognitive deficits found in individuals who do not use cannabis and later develop dementia.

Here is the central question only more study over time will answer: Will cannabis use, in combination with cognitive deficits that normally come with advancing age, increase the rate of dementia late in life for regular cannabis users?

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