A study published in Addiction investigated the effects of cannabis and tobacco use on brain volume, finding associations between both substances and reductions in specific brain regions when used separately. Researchers noted that while consistent brain changes were linked to tobacco use, cannabis research remains too limited to draw firm conclusions.
The Global Landscape of Cannabis and Tobacco Use
In 2022, approximately 228 million people, or 4.4% of the global population, used cannabis. The increasing potency of cannabis products is associated with higher risks of mental health disorders and addiction.
Tobacco use is even more widespread, with an estimated 1.18 billion users in 2020, representing roughly 30% of the world's population. This widespread use contributes to over eight million deaths annually.
Some individuals engage in co-use of both substances, either simultaneously (e.g., in spliffs) or separately within short time frames, thereby exposing the brain to distinct neurobiological effects.
How Nicotine and THC Impact the Brain
Nicotine, the primary psychoactive compound in tobacco, acts on nicotinic acetylcholine receptors located in various brain regions.
Tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, interacts with cannabinoid type 1 (CB1) receptors, which are abundant throughout many areas of the brain.
Key Study Findings on Brain Volume
The systematic review and meta-analysis revealed specific associations between substance use and brain volume changes.
Cannabis UsersCross-sectional analyses consistently indicated smaller amygdala volumes in cannabis users. However, longitudinal studies presented inconsistent evidence, with observed reductions primarily found in users with or at risk for schizophrenia.
Tobacco UsersNo Mendelian randomization (MR) studies were available to assess causal effects of cannabis on brain volume, highlighting a gap in current research.
Cross-sectional analyses for tobacco users revealed smaller volumes in multiple brain regions. These included the amygdala, insula, pallidum, and total grey matter volume. Longitudinal studies further identified reduced total gray matter volume in tobacco smokers.
Biological Explanations and Co-UseMR studies provided stronger evidence, linking smoking initiation to decreased amygdala volumes and higher daily cigarette consumption to reduced hippocampal volume.
These observed brain changes may contribute to the psychoactive effects of the drugs, aligning with their known receptor distribution in the brain. Tobacco combustion may also induce oxidative stress and inflammation, potentially leading to neuronal death. While cannabis combustion also produces toxins, cannabidiol (CBD) might offer counteracting anti-oxidant and anti-inflammatory effects.
Research on the co-use of both drugs is notably limited; the study found only one cross-sectional analysis, which showed no significant differences in hippocampal volume compared to controls or nicotine-only users.
Limitations and Implications for Public Health
This study, the first systematic review and meta-analysis of its kind, incorporated various study designs but also had several limitations. Cross-sectional meta-analyses cannot establish causal inferences, and observed associations may be influenced by confounding factors.
The study used binary classifications of use, overlooking critical details such as drug potency and usage severity. Furthermore, its primary focus on cortical volume might have missed the significance of changes in brain surface area relative to cortical thickness.
These findings suggest that tobacco use is associated with reductions in specific brain regions involved in memory and emotional processing, with weaker and less consistent evidence for cannabis use.
The consistency of tobacco-related findings across different study designs strengthens their implications. The authors underscore the need for more longitudinal research, especially concerning the combined use of cannabis and tobacco, as global cannabis consumption continues to rise. The observations support public health messaging regarding the potential harms of these substances to the brain.