Ultra-heavy cannabis users with no other substance use showed elevated psychotic traits and cognitive deficits | Research

The takeaway

In a unique population of exclusive, ultra-heavy cannabis users averaging over 30,000 lifetime exposures, researchers found significantly higher schizotypal scores and worse cognitive performance compared to matched non-using controls.

Psychosis researchers, cognitive scientists, anyone interested in long-term cannabis effects

30,000+ lifetime cannabis exposures; no other substance use; significantly elevated psychotic traits

What the researchers found

Cases averaged 30,000+ lifetime cannabis exposures with no other substance use. They had significantly higher SPQ scores (24 vs. 13, p = 0.031) and lower composite cognitive scores (-0.23 vs. +0.28, p = 0.03) than controls. Moderate to large effect sizes were noted for attention, psychomotor speed, working memory, cognitive flexibility, visuospatial processing, and verbal memory. A subsample showed worse scores than their own non-using siblings.

Why it matters

This study addresses a major limitation in cannabis research: confounding from other substance use. By studying a population that exclusively uses cannabis, the findings are more directly attributable to cannabis itself.

The numbers in context

15 cases, 12 controls. Mean SPQ: 24 vs. 13 (p = 0.031). Composite cognition: -0.23 vs. +0.28 (p = 0.03). Cases averaged 30,000+ lifetime cannabis exposures. Moderate to large effect sizes across cognitive domains.

How the study worked

Cross-sectional study of 15 ultra-heavy cannabis users from a unique cultural group where cannabis use is central but all other substances (tobacco, alcohol) are forbidden. Compared with 12 matched controls from the same community. Schizotypal Personality Questionnaire and culture-neutral cognitive battery administered.

What this study cannot tell us

Very small sample; cross-sectional design; the unique cultural context may limit generalizability; cannot determine whether pre-existing differences led to cannabis use patterns.

How to read the evidence

Very small but uniquely controlled sample addressing the critical confound of polysubstance use.

When this study was published

Published in 2020.

The bigger picture

While most cannabis studies struggle with polysubstance confounding, this unique population provides unusually clean evidence that heavy, chronic, early-onset cannabis use is associated with psychosis-relevant traits and cognitive deficits.

Questions still open

Would similar effects be found at lower levels of cannabis exposure? Is the cultural context protective in ways that could be studied?

Common questions

What makes this study special?

Most cannabis studies cannot separate cannabis effects from other substance use. This study examined a cultural group where cannabis is central to daily life but all other substances (including tobacco and alcohol) are forbidden, providing unusually clean evidence.

How heavy was their cannabis use?

The 15 cases averaged over 30,000 lifetime cannabis exposures, beginning early in life. This is far heavier than typical study populations and represents an extreme end of the use spectrum.

Read the original research

Characterizing psychosis-relevant phenomena and cognitive function in a unique population with isolated, chronic and very heavy cannabis exposure.

Psychological medicine, 50(14), 2452-2459

View on PubMed ↗Publisher’s paper ↗

Citation

D'Souza, Deepak Cyril; Ganesh, Suhas; Cortes-Briones, Jose; Campbell, Michael H; Emmanuel, Maisha K. (2020). Characterizing psychosis-relevant phenomena and cognitive function in a unique population with isolated, chronic and very heavy cannabis exposure.. Psychological medicine, 50(14), 2452-2459. https://doi.org/10.1017/S0033291719002721