Cannabis and tobacco—whether used on their own or together—can affect the
brain's structure. A recent study involving
systematic review and meta-analysis of more than 103 studies found that both
substances are linked to reduced volumes in specific brain regions.
The researchers observed that regular cannabis users tend to
have a smaller amygdala—an almond-shaped structure deep in the brain that helps
regulate emotions and the fight-or-flight response.
At the same time, regular tobacco use was associated with volume variations
across multiple brain regions, including smaller volumes in the amygdala,
insula, a region that plays a key role in self-awareness and emotion, and
pallidum, which is crucial for maintaining proper movement and motivation.
The analysis also revealed that, over the years, the changes in brain
structures become even clearer: compared to non-users, people who smoke
tobacco saw a faster decline in total gray matter volume. This trend might be
concerning as gray matter volume is a key marker of brain health and closely
tied to both cognitive performance and mental well-being. A decline in its
volume can therefore have lasting effects on how we think, feel, and function.
The findings are published in Addiction.
Triangulating brain volume variation evidence
One in 20 adults across the globe use cannabis, that is, around 228 million
people globally, and the number is seeing an upward trend. Tobacco is smoked by
nearly 1.8 billion people, which is
nearly 30% of the current human population. This habit takes over 8 million
lives in just a year. Both these substances have psychoactive effects where
they can alter how one feels—in other words, how they impact the brain.
In cannabis, the main mind-altering compound is tetrahydrocannabinol (THC),
which acts on cannabinoid type 1 receptors, known as CB1 receptors, in the
brain. These receptors play a major role in controlling pain, anxiety, and
appetite.
The main psychoactive component of tobacco, nicotine, acts on
nicotinic acetylcholine receptors, known as nAChRs. These receptors are found
throughout the brain and play an important role in how nerve cells communicate
and the brain's ability to adapt and learn.
Studies have time and again found that both cannabis and tobacco can have
negative effects on the brain, but not much was known about their impact on
brain volume.
To get a better understanding of how cannabis and tobacco affect the brain,
the researchers of this study conducted a massive review that analyzed 103
independent studies involving more than 72,000 people.
It combined three types of scientific evidence: cross-sectional studies
that compared users and non-users at a single point in time, longitudinal
studies that followed the same people over years to track brain changes, and
Mendelian randomization studies that used genetic data to test whether
substance use actually causes these changes.
The team found that cannabis and tobacco use are both linked to reductions
in brain volume, affecting different regions in the brain—amygdala, insula,
pallidum and gray matter. Genetic analyses also suggested that smoking more
cigarettes per day was strongly linked to a smaller hippocampus, the brain
region that handles our memory and learning.
By pinpointing exactly where brain volume shrinks, this study gives health care professionals something specific for a show-and-tell. Instead of vague warnings, they can have clearer, more relatable conversations with patients, especially young and vulnerable groups, about how these substances are shaping their brains and their long-term health.
Source: Cannabis and tobacco use linked to smaller brain volume
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