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Research & Development Adverse effects, Medical research, Neurology & psychology

A Ticking Time Bomb?

In a recent study, a team of international researchers teamed up to examine the long-term effects of cannabis use (1). The data originate from the Dunedin Multidisciplinary Health and Development Study, which has followed a group of 1,037 children born in 1972–73 in Dunedin, New Zealand, from birth to now. IQ was tested at ages seven to 11 (before cannabis use) and again at age 45, after some study members had used cannabis for many years.

The research was prompted by the confluence of two trends: the growth of the aging population and the record high rates of cannabis use among today’s older adults. Specific cognitive abilities – such as learning, memory, executive function, processing speed, perceptual reasoning, verbal comprehension, and motor function – were tested at age 45. The researchers also assessed hippocampal volume using magnetic resonance imaging of the brain. Results revealed lower cognitive abilities and hippocampal volume in long-term cannabis users. We spoke to Madeline Meier, first author of the study, about these findings.

Could you briefly summarize your main findings?

 Long-term cannabis users in this study were using weekly or more often as 45-year-olds and had also used cannabis weekly or more often when they were assessed at previous ages (18, 21, 26, 32, and 38). This population showed IQ decline and poorer learning and processing speed in midlife relative to their childhood IQ. People who knew them well described them as having memory and attention problems. Cognitive deficits were unique to long-term cannabis users; deficits were not apparent – or less apparent – among long-term tobacco users, long-term alcohol users, midlife recreational cannabis users, and cannabis quitters (who had used cannabis in the past but had stopped by age 45).

These deficits were not explained by persistent tobacco, alcohol, or other illicit drug use, or by childhood social class, low childhood self-control, or family substance dependence history. Associations were also not explained by recent cannabis use the night before testing. Finally, long-term cannabis users displayed smaller hippocampal volume in midlife, though this was not the mechanism underlying the association between long-term cannabis use and cognitive deficits.

What are the implications of these findings?

Our study demonstrates the importance of “before and after” IQ testing. Few studies have ruled out the possibility that poorer cognitive test performance among cannabis users predates their cannabis use initiation. We showed that, regardless of IQ test performance in childhood, long-term cannabis users performed worse than others when tested in midlife.

We also highlighted the importance of cognitive deficits and smaller hippocampal volume in midlife. Previous studies have shown that heavy cannabis users exhibit mild cognitive deficits and smaller hippocampal volume relative to non-users. However, most of those studies were in adolescent and young adult cannabis users. It is unclear whether the subtle cognitive and brain differences observed in young cannabis users might be larger in midlife and older adult cannabis users with longer histories of use.

Our study is one of only a few to examine cognitive functioning and hippocampal volume in midlife or older adult cannabis users. Midlife is significant because previous research shows that cognitive deficits and greater hippocampal atrophy in midlife are risk factors for dementia. These deficits were generally mild to moderate, but still comparable to the midlife cognitive deficits of individuals who went on to develop dementia in the Atherosclerosis Risk in Communities Study (2).

Do you have a call to action as a result of these findings?

The findings suggest that long-term cannabis users should stop – or at least reduce – their cannabis use before age 45. Approximately 8 percent of the 1,037 individuals born in one year in Dunedin became “long-term” users who initiated cannabis use when young and continued using regularly into their fifth decade. This is quite a significant proportion of the population and further research is needed to ascertain whether long-term cannabis users show elevated rates of dementia in later life.

Any plans for further research?

The research team is now examining associations between long-term cannabis use and comprehensive magnetic resonance imaging measures of global and regional grey and white brain matter.

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  1. MH Meier et al., Am J Psychiatry, [Online ahead of print] (2022). DOI: 10.1176/appi.ajp.2021.21060664.
  2. DS Knopman et al., Alzheimer’s & Dementia, 14, 1406 (2018). DOI: 10.1016/j.jalz.2018.03.011.
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