A comprehensive meta-analysis reveals that tobacco use is consistently linked to reduced brain volume in areas governing memory and emotion, while evidence regarding cannabis remains weaker and less conclusive.
A new systematic review and meta-analysis published in the journal Addiction has found that tobacco smoking is consistently associated with significant reductions in total grey matter and key brain regions like the amygdala and hippocampus. While cannabis use showed some association with smaller amygdala volumes, the evidence is currently too limited to draw firm causal conclusions, highlighting a critical need for longitudinal research on both separate and co-use of these neuroactive substances.
The Global Burden of Neuroactive Substances
Tobacco and cannabis are two of the most widely consumed neuroactive drugs in the world. In 2020, an estimated 1.18 billion people (roughly 30% of the global population) used tobacco, making it the leading preventable cause of death worldwide. Concurrently, cannabis use is rising, with approximately 228 million users in 2022. The increasing potency of modern cannabis products has raised new concerns regarding mental health and addiction.
Because these substances are often used together—either in spliffs or within short time windows—understanding their distinct neurobiological effects is critical for public health. Nicotine acts on nicotinic acetylcholine receptors, while THC (the primary psychoactive compound in cannabis) binds to CB1 receptors. Both receptor types are abundant throughout the brain, but their structural impacts appear to differ significantly.
Comparative Brain Volume Reductions
The study utilized cross-sectional, longitudinal, and Mendelian randomization (MR) studies to test causal associations. The following table summarizes the structural brain changes observed in users of both substances.
| Substance | Brain Regions Affected | Strength of Evidence |
|---|---|---|
| Tobacco | Amygdala, Insula, Pallidum, Hippocampus, Total Grey Matter | Strong. Supported by MR and longitudinal studies. |
| Cannabis | Amygdala | Weak/Inconsistent. Lacks MR studies; mostly cross-sectional. |
Tobacco: Consistent Evidence of Neurological Shrinkage
The evidence regarding tobacco is stark and consistent across multiple study designs. Cross-sectional analyses revealed smaller volumes in the amygdala, insula, pallidum, and total grey matter among smokers. Longitudinal studies confirmed these findings, showing a progressive reduction in total grey matter compared to non-smokers.
Crucially, the higher-quality MR studies provided evidence of causality. They linked smoking initiation to decreased amygdala volumes and associated higher daily cigarette consumption with reduced hippocampal volume. Researchers hypothesize that the combustion of tobacco triggers oxidative stress and inflammation, which may directly cause neuronal death in these critical areas responsible for emotion and memory processing.
Cannabis: Inconclusive Data and Potential CBD Mitigation
The neurobiological impact of cannabis remains less clear. Cross-sectional analyses did show smaller amygdala volumes among users. However, longitudinal evidence was inconsistent, with reductions primarily observed in users who already had schizophrenia or were at risk for it. Notably, there were no MR studies available to assess the causal effects of cannabis on brain volume.
While cannabis combustion also produces toxins, researchers suggest that the presence of cannabidiol (CBD)—which possesses anti-oxidative and anti-inflammatory properties—might counteract some of the neuronal damage typically caused by smoke inhalation.
Expert Verdict: The Need for Co-Use Research
This meta-analysis is the first of its kind to assess the structural brain impacts of these substances both separately and combined. The findings strongly support public health messaging regarding the neurological harms of tobacco. However, the study exposes a massive gap in current scientific literature: there are virtually no longitudinal studies examining the co-use of tobacco and cannabis. As global cannabis consumption continues to rise, understanding how these two substances interact within the brain is an urgent public health imperative. Until more robust data is available, the structural risks of high-potency cannabis remain a critical unknown.
- Journal reference: Sawyer, K., Freeman, T. P., Skumlien, M., et al. (2026). Associations of cannabis use, tobacco use and co-use with brain volume: A systematic review and meta-analysis. Addiction. DOI: https://doi.org/10.1111/add.70361. https://onlinelibrary.wiley.com/doi/10.1111/add.70361