Canada has become the second country in the world to formally legalise cannabis for recreational use, following in the footsteps of Uruguay, and analysts suggest the Canadian cannabis market will be worth more than $4.2bn annually. This legal change, alongside its decriminalized status in Portugal and the Netherlands, reflects the change in public perception of the drug. Recent reports show the number of adolescents who believe that regular cannabis use is harmful is declining, but cannabis has become the dominant reason for young people to seek addiction treatment across Ireland. Recent research has aimed to measure the impact of cannabis on brain function, but the extent of its effects remains poorly understood.

There are a variety of active substances in cannabis, but the two most prominent are D9-tetrahydrocannabinol (THC) and cannabidiol (CBD). THC is the psychoactive cannabinoid that produces the “high”, and it effects memory, motor functions, pain response, and sleep. THC interacts with the cannabinoid receptors on nerve cells. When activated by THC, nerve cells are less likely to fire messages to their neighbouring cells, and this triggers the brain’s reward system to produce pleasure chemicals such as dopamine. The way in which CBD interacts with the body is not fully known due to its complexity, but CBD is not psychoactive, and it has been shown to have anti-inflammatory properties. Some research indicates it counteracts the disordered thinking and anxiety caused by THC.

The effect of cannabis on creativity has been the subject of several studies recently, to bring some clarity to the situation: On one hand, some cannabis users claim, based on experience, that the drug leads to more unusual and original thoughts, with more than 50% of users reporting heightened creativity while high in a 2003 study. “[Marijuana] would make me relaxed and creative,” Steve Jobs once said, and there is no shortage of great musicians who wrote music while high. However, the results from experiments have not backed this up, but the way in which researchers define and measure creativity has made it difficult to take into the laboratory.

“Recent reports show the number of adolescents who believe that regular cannabis use is harmful is declining, but cannabis has become the dominant reason for young people to seek addiction treatment across Ireland”

A recent study aimed to settle the debate by studying how cannabis affects creativity in regular users. To quantify creativity, the researchers divided the problem into divergent and convergent thinking. Divergent thinking refers to brainstorming, where one is seeking many solutions to a problem. (e.g. “How many words can you list that rhyme with ‘boat’?”). In contrast, convergent thinking involves trying to find one specific solution to a problem (e.g. “Which of the following words rhymes with ‘boat’: observer, goat, university”).

These two types of creative thinking relate differently to the levels of dopamine in the brain, where divergent thinking is expected to peak with medium dopamine levels. THC has been shown to indirectly induce dopamine release in the brain so, coupled with the fact that THC is known to reduce one’s inhibitions, it was theorized that THC would have a beneficial effect on creativity stemming from divergent thinking.

The results were clear-cut: The researchers found that high doses of cannabis for regular users was detrimental to their divergent thinking, and less potent cannabis had no effect, positive or negative, on this aspect of creativity. The researchers note that the feeling of heightened creativity from the user’s point of view may be illusory, concluding that “a joint may not be the best choice when in need of breaking the writer’s block, and smoking several of them might actually be counterproductive.” However, one drawback of the study is that they looked at the effects on heavy users, not non-users or even casual users.

Not only has cannabis not been shown to lend a creative hand, several large studies go further and highlight the detrimental effects of long-term use on the brain. Professor Zalesky from the University of Melbourne examined MRI scans of marijuana users and found that the neural connections between the two hemispheres were impaired, relative to the non-users. As well as this, the earlier someone started smoking, the more pronounced the impairments were. A New Zealand study followed over a thousand people from birth until the age of 38 and found that those who started smoking cannabis during their teenage years had a lower IQ in their thirties than they did in their childhood. As in the study by Zalesky, those who were youngest when they started smoking and those who smoked most heavily showed the greatest decline in cognitive performance.

“The researchers note that the feeling of heightened creativity from the user’s point of view may be illusory”

It is unclear the extent to which abstinence can alleviate the negative long-term effects, but we know that the neuroplasticity of the brain makes it remarkably good at compensating for damage. While a 2014 study found that cognitive impairments associated with cannabis use can persist even after abstinence, a different experiment found no cognitive defects in former users after only three months of going cold-turkey.

Perhaps the most serious side-effect is the evidence linking cannabis use to an increased risk of developing psychosis. A Swedish study of 45,000 individuals found that those who used high amounts of cannabis were six times more likely to develop schizophrenia. The higher the THC content, the greater the risk, and Dr Johnny Connolly, University of Limerick, claims that this can be seen in Ireland, where the THC of cannabis and the rate of drug induced psychosis are both rising.

Cannabis use may provide short-term relief from anxiety because THC binds to cannabinoid receptors in the amygdala, reducing the brain’s threat response. However, while some suggest it as a treatment for mental disorders like anxiety, the consensus of medical professionals is that using cannabis will ultimately exacerbate any underlying conditions. The plant is not without any medicinal value though, and some benefits for treating specific conditions have already been identified. For example, there is evidence that THC and CBD medications can help people diagnosed with multiple sclerosis, and CBD can reduce seizures in children with Dravet syndrome.

While the question of prevention and prohibition versus education and decriminalization is not straightforward, it is clear that legislation is currently outpacing our scientific understanding of the drug. Establishing the positive and negative effects of cannabis should be a research priority as the information is needed to shape the public debate. In many respects, the Canadian situation should provide a novel research opportunity for this.