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The Effect of THC on the Dopamine System

Wednesday 21 December 2016 12:14pm

Cannabis has been under the microscope for decades with evidence for it being both a potentially harmful and beneficial substance. With its legalisation for medicinal purposes, cannabis has been used to treat chronic pain, loss of appetite, seizures, and a number of other illnesses with varying levels of success. However, a recent review of cannabis research, published in the prestigious scientific journal Nature, has revealed that, while it may have medical benefits, repeated casual use may have harmful effects on the brain.

The researchers examined studies which specifically related to the effects of THC, the psychoactive component of cannabis, on the dopamine system of the brain. THC content of medicinal cannabis products varies according to the illness being treated however, because of the psychoactive properties, it is the key ingredient that casual users are interested in. Of particular relevance here is the fact that the THC content of cannabis on the street has increased at least 3-fold in the past 20 years. In their examination of the research, this team found that THC impacted the brain’s dopamine system in a variety of different ways.

Dopamine has a number of different actions in the brain. It is involved in basic functions like our motor system and reward system, and higher cognitive functions including decision making and emotional regulation. These dopamine systems are kept in check by feedback mechanisms, which make sure that they’re working effectively and not being under or over-stimulated.

THC alters the feedback processes of cells in dopamine systems. It affects the cells by binding to a cannabinoid receptor, which increases the creation and release of dopamine. In a single use case this produces short-term effects: stress relief, appetite stimulation, working memory problems, dysfunction of executive decision making, and loss of motivation. With chronic use the effect on the dopamine system becomes more severe and begins to impact the user at all times, not just when they are ‘high’.

With sustained use the brain adapts. The dopamine system attempts to desensitize to counterbalance these floods of dopamine. This is called blunting, as it reduces the precision of the systems it affects. Rewards become less rewarding, motor function becomes less precise, memory is impaired, motivation is reduced, and on top of that it becomes more difficult to emotionally regulate and users will begin to experience more negative emotions. The effects of chronic use are also exacerbated by normal problems like stress and sleeplessness, resulting in a wide-reaching but relatively subtle loss of function.

It isn’t all bad news. The research suggests that with time these effects can be reversed. Once an individual stops ingesting THC their brain begins to recover, eventually restoring the dopamine system to the level of sensitivity it had prior to chronic THC use. It’s important to note that THC isn’t killing brain cells, it’s just changing how they work, and so long as those cells survive there is hope for recovery. There is always the risk, however, that changes caused by THC will set-off a chain of events and result in pathological brain changes.

Can marijuana kill you? No, not at normal dose levels, but depending on the amount of THC that the marijuana contains, it can be harmful. For those using cannabis for medical reasons this has to be weighed against the potential benefits, as with all medications. Luckily, for some of those conditions THC content can be reduced without reducing the effectiveness of treatment.

Cannabis is not a neutral substance. It can be beneficial and it can be harmful. As more research emerges, however, it becomes clearer and clearer that sustained recreational use of marijuana, especially when it has the high levels of THC now available, can produce long-term changes in brain function.


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