Tetrahydrocannabinol, or THC, is the main psychoactive compound in the cannabis plant. When a person consumes cannabis, this chemical is primarily responsible for the “high” and other psychological effects they experience. These effects are not random; they result from THC interacting with a complex network of signals and receptors in the human brain. This interaction alters normal brain function, leading to changes in perception, thought, and mood.
How THC Interacts with the Brain’s Endocannabinoid System
The human brain has a widespread cell-signaling network called the endocannabinoid system (ECS). This system regulates a range of functions, including mood, pain, appetite, and memory. The ECS functions through a “lock and key” mechanism, where natural chemicals called endocannabinoids bind to specific cannabinoid receptors. The two main receptors are CB1 and CB2, with CB1 receptors being abundant in the brain.
THC’s chemical structure is similar to the body’s own endocannabinoids, allowing it to activate these same CB1 receptors. However, THC acts like a stronger and more persistent key than the body’s natural ones. Because THC overwhelms the ECS, it disrupts its ability to regulate communication between neurons.
This disruption is pronounced because CB1 receptors are highly concentrated in brain regions responsible for complex functions. These areas include those governing thought, memory, coordination, and time perception. By binding to these receptors, THC temporarily takes over the regulatory role of the ECS, leading to effects that alter a person’s immediate experience.
Immediate Effects on Perception and Cognition
The psychoactive effects of THC are a consequence of its interference with communication in specific brain regions. One of the most affected areas is the hippocampus, which is integral to forming new memories. THC’s disruption of the hippocampus is why individuals often experience short-term memory impairments, finding it difficult to recall recent events. This interference can also lead to the creation of false memories.
Motor control and physical coordination are also affected. These functions are governed by the cerebellum and the basal ganglia, two brain regions dense with CB1 receptors. When THC activates these receptors, it can lead to slowed reaction times and difficulty with balance. This is why activities that require fine motor skills become challenging under the influence of cannabis.
Higher-level thought processes are managed by the cerebral cortex, which is responsible for decision-making and problem-solving. THC’s interaction with this area leads to altered thinking, a distorted sense of time, and changes in sensory perception. It can also impair judgment and make complex tasks difficult to perform.
The amygdala, which processes emotions like fear and anxiety, is also sensitive to THC. Depending on the dose and the individual’s psychological state, THC’s influence on the amygdala can have varied outcomes. For some, it may reduce feelings of anxiety, while for others, it can induce paranoia or panic.
The Connection to Dopamine and Reward Pathways
While THC’s primary action is on the endocannabinoid system, it also indirectly influences the brain’s reward pathways. This system is driven by dopamine, a neurotransmitter associated with pleasure, motivation, and reinforcement. All known drugs of abuse, including THC, cause an increase in dopamine levels in the brain’s reward circuit, particularly in a region called the nucleus accumbens.
THC does not trigger the release of dopamine directly. Instead, by activating CB1 receptors, it alters the activity of neurons that regulate dopamine-releasing cells. THC can reduce the release of an inhibitory neurotransmitter called GABA. Since GABA normally acts as a “stop” signal for dopamine neurons, inhibiting GABA allows these neurons to release more dopamine than they normally would.
This resulting surge of dopamine creates the sense of euphoria and relaxation that many people seek when using cannabis. The brain’s reward system learns to associate cannabis use with this pleasurable feeling, reinforcing the behavior. Over time, this reinforcement can contribute to a cannabis use disorder as the brain craves the substance to replicate the rewarding experience.
Potential Long-Term Brain Alterations
The consequences of chronic cannabis use can extend beyond the immediate high, leading to lasting changes in the brain’s structure and function. The adolescent brain is particularly vulnerable to these alterations. The brain continues to develop until about age 25, and THC exposure during this period can disrupt the maturation of the prefrontal cortex, which may be associated with later difficulties in memory and learning.
In the adult brain, chronic exposure to high THC levels can cause the brain to adapt, diminishing the drug’s effects. The brain compensates for the constant overstimulation by reducing the number of available CB1 receptors, a process known as downregulation. This adaptation means a person needs to use more THC to achieve the same feeling, a phenomenon known as tolerance. This is also linked to withdrawal symptoms like irritability when use is stopped.
Research into the long-term effects is ongoing, but some studies have linked heavy use to changes in brain structure, such as reduced volume in the hippocampus. Furthermore, there is a documented association between high-potency cannabis use and an increased risk of psychosis, particularly in individuals with a genetic predisposition. These findings suggest that repeated interference with the brain’s natural systems can have lasting consequences.