Cannabis, commonly known as marijuana, is a plant-derived substance used for medicinal and recreational purposes. As its legal status changes, a prominent public concern revolves around its potential impact on cognitive abilities, particularly intelligence quotient (IQ). Exploring the scientific evidence behind this claim is important for understanding how cannabis interacts with the human brain and its lasting effects.
Understanding the Research on Cannabis and Cognitive Function
Research on cannabis use and cognitive function, including IQ, presents a complex picture. Epidemiological studies often show an association between regular cannabis use and poorer cognitive performance. For instance, some studies indicate that frequent cannabis use in youth is linked to a small but measurable decline in IQ, averaging around 2 points.
Longitudinal studies track individuals over time, assessing cognitive changes before and after cannabis exposure. One study noted an average decrease of 4.1 IQ points in heavy current users compared to non-users. However, other twin studies suggest that shared familial factors, including genetics, might underlie the association between adolescent cannabis use and IQ decline, rather than cannabis being the sole cause. Some research indicates that lifetime cannabis use may not be associated with later cognitive decline in adults, suggesting that effects might not persist into older age.
How Cannabis Interacts with the Brain
Cannabis primarily affects the brain through compounds called cannabinoids, like delta-9-tetrahydrocannabinol (THC), its main psychoactive component. These cannabinoids interact with the body’s natural endocannabinoid system (ECS). The ECS is a cell-signaling network throughout the brain and nervous system that regulates physiological and cognitive processes, including memory, learning, and executive function.
The ECS includes endocannabinoids (molecules produced by the body), cannabinoid receptors (CB1 and CB2), and enzymes. THC binds to CB1 receptors, which are abundant in brain regions involved in memory and cognitive function, such as the hippocampus and prefrontal cortex. This interaction can disrupt the normal functioning of the ECS, potentially affecting memory formation, information processing, and other cognitive abilities.
Why Adolescence is a Critical Period
Adolescence is a period of brain development, making it vulnerable to external influences like cannabis use. The human brain continues to mature until around age 25, with remodeling occurring in areas like the prefrontal cortex. This region is responsible for executive functions, decision-making, and higher-level cognitive abilities that contribute to IQ.
During adolescence, the brain undergoes processes such as synaptic pruning, which eliminates unused neural connections, and myelination, which improves nerve impulse speed. Cannabis use during this phase can interfere with these developmental processes. Studies suggest that moderate-to-heavy cannabis use in adolescents is associated with reduced thickness in the prefrontal cortices, with greater use leading to more cortical thinning. This disruption may lead to more pronounced and lasting cognitive effects compared to cannabis use initiated in adulthood.
What Happens After Discontinuing Use
Whether cognitive effects from cannabis use are reversible upon abstinence is a common concern. Research suggests that some cognitive functions can improve after discontinuing cannabis use, particularly in the short term. Studies show that memory functions, especially the ability to learn and recall new information, can improve within one month of abstinence among regular adolescent and young adult users.
Improvements in verbal learning and memory are observed after one to two weeks of abstinence, while psychomotor speed may recover after about a month. However, the extent of recovery varies and depends on factors such as age of onset, duration, and frequency of cannabis use. While some deficits, like those in attention, may persist for several weeks, long-term heavy use, particularly beginning in adolescence, can lead to more persistent cognitive impacts that do not recover as readily.