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Many people experiment with cannabis during their adolescence and early adulthood, and new research shows this is a particularly dangerous age because the brain is still developing. A recent study reported in the Oxford University Press found compelling evidence that brain reacts differently to cannabis exposure that commences during adolescence compared with adulthood.

The aim of the study was to examine the “white matter” pathways within the brain to see if they are changed through long-term heavy cannabis use. It was also hypothesized that the earlier someone started heavy and regular cannabis use the more severe the affect would be on these pathways in the brain.

White matter is the tissue through which messages pass between different areas of gray matter within the brain. Using a computer network as an analogy, the gray matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together.

There are three different kinds of tracts, or bundles of axons which connect one part of the brain to another and to the spinal cord, within the white matter: projection tracts that extend vertically between higher and lower brain and spinal cord centers; commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures; and association tracts that connect different regions within the same hemisphere of the brain.

To measure the effect of cannabis on the “white matter” pathways, the study looked at long term habitual users of cannabis with a minimum usage of twice a month for the past 3 years (although most recruited in the study had substantially greater use than this) and compared them with healthy non-users. In-depth brain imaging and brain connectivity mapping techniques were performed in each of the 59 cannabis users with longstanding histories of heavy use and the 33 healthy non-users who served as controls.

After examining the habitual heavy cannabis users, researchers found the axonal pathways were impaired in the right fimbria of the hippocampus, splenium of the corpus callosum and commissural fibers. It is also important to note that all of these areas that were impaired have an abundance of cannabinoid receptors.

The amount of damage to these pathways was also directly associated with the age at which regular cannabis use commenced. This resulted in long term users having greater instance of anxiety and depressive symptoms and lower Global Assessment of Functioning scores (a measurement of social, occupational and psychological functioning).

This association presents compelling evidence for white matter reacting differently to cannabis exposure commencing during adolescence compared with adulthood, most likely due to the high concentration of cannabinoid receptors contained within structures, such as the corpus callosum and fornix during adolescence.

These results suggest that long-term cannabis use is hazardous particularly to white matter in the developing brain of adolescents and young adults. Damage to these vital pathways during brain development may later lead to cognitive impairment and vulnerability to psychosis, depression and anxiety disorders, all of which are significant public health concerns. White matter alterations have been associated with various functional and clinical outcomes in schizophrenia, including illness, symptomatic and cognitive measures.

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