Researchers show how marijuana produces radically different effects in different individuals

How marijuana affects us may come down to the region of the brain it is acting on. Researchers at Western University have shown that different regions of the brain produce radically different effects from marijuana use.

The psychological effects of marijuana can differ between individuals – some experience highly rewarding effects which may lead to dependence on the drug, while others may experience negative psychiatric side effects including paranoia, cognitive problems or an increased risk of developing schizophrenia. 

“Until now, it was unknown which specific regions of the brain were responsible for these highly divergent effects of marijuana,” said Steven Laviolette PhD, Professor at Western’s Schulich School of Medicine & Dentistry. “Translational rodent research performed in our lab has identified highly specific target regions in the brain that seem to independently control the rewarding, addictive properties of marijuana versus the negative psychiatric side-effects associated with its use.” 

The study, led by Laviolette and postdoctoral fellow, Christopher Norris, PhD, was published today in Scientific Reports. By looking at THC’s effect on a rat brain, the researchers demonstrate that THC, the main psychoactive compound in marijuana, can produce highly rewarding effects in a region of the brain called the nucleus accumbens, but only in the most anterior regions of this structure. 

The study showed that THC in this brain area not only produced highly rewarding effects in and of itself, it was able to strongly increase the addictive properties of opioid drugs like morphine and increase reward-related activity patterns in the neurons. In contrast, THC in the posterior area of this specific brain region produced highly aversive effects, including increased schizophrenia-related cognitive and emotional symptoms and produced patterns of neuron activity similar to those found in schizophrenia. 

“These findings are important because they suggest why some people have a very positive experience with marijuana when others have a very negative experience,” said Norris. “Our data indicate that because the reward and aversion are produced by anatomically distinct areas, the different effects between individuals is likely due to genetic variation leading to differential sensitivity of each area.” 

The researchers say these findings reveal critical new insights into how marijuana can produce such highly diverse psychological effects in different individuals. They also suggest that the specific area of an individual’s nucleus accumbens that is more sensitive to THC, might be a critical indicator of whether they experience positive or negative side-effects from marijuana exposure.

MEDIA CONTACT: Stephen Ledgley, Senior Media Relations Officer, Western University, 519.661.2111 x85283,

Western University delivers an academic experience second to none. Since 1878, The Western Experience has combined academic excellence with life-long opportunities for intellectual, social and cultural growth in order to better serve our communities. Our research excellence expands knowledge and drives discovery with real-world application. Western attracts individuals with a broad worldview, seeking to study, influence and lead in the international community.

The Schulich School of Medicine & Dentistry at Western University is one of Canada’s preeminent medical and dental schools. Established in 1881, it was one of the founding schools of Western University and is known for being the birthplace of family medicine in Canada. For more than 130 years, the School has demonstrated a commitment to academic excellence and a passion for scientific discovery.