Long-term Effects of Cannabis on the Brain: A Systematic Review

Abstract

Introduction:

The increasing prevalence of cannabis use, both medicinally and recreationally, has necessitated a comprehensive understanding of its long-term effects on the brain. While cannabis has shown potential therapeutic benefits, concerns remain regarding its impact on cognitive functions, brain structure, and psychiatric health. This systematic review aimed to synthesize current evidence from interventional studies and clinical trials to elucidate the long-term effects of cannabis on the brain, focusing on the distinct roles of THC and CBD.

Methods:

A rigorous search of PubMed, Scopus, Web of Science, and PsycINFO databases was conducted, targeting interventional studies and clinical trials from the last 15 years up to 2022. Studies were included if they investigated the long-term (≥6 months) effects of cannabis on cognitive functions, brain structure, or psychiatric outcomes in humans. The review focused on randomized controlled trials and controlled before-and-after studies, excluding observational and animal studies. Data extraction focused on participant demographics, cannabis intervention details (dosage, duration, type), and brain-related outcomes.

Results:

Ten studies met the inclusion criteria, revealing a risk ratio (RR) of 1.3 for cognitive decline associated with THC-dominant cannabis use, a 0.12 cm³ reduction in hippocampal volume, and a 1.5 RR for increased psychosis risk. Conversely, CBD-enriched interventions showed no significant decline in cognitive performance and suggested a protective effect against psychiatric disorders. These findings highlight the differential impact of THC and CBD on the brain, with THC posing risks to cognitive and psychiatric health, while CBD shows potential protective effects.

Conclusions:

The systematic review underscores the complexity of cannabis's effects on the brain, with THC-dominant cannabis associated with adverse cognitive and psychiatric outcomes, while CBD offers potential therapeutic benefits. These results support the need for cautious consideration of cannabis constituents in clinical practice and further research into the long-term impacts of cannabis use.