Long-term marijuana use has been linked to a number of mental health conditions, including schizophrenia, anxiety, and depression, but the mechanisms underlying this association have been unclear.
Given the increased legalization of marijuana for medicinal and recreational purposes, researchers are keen to learn more about how the drug affects the brain.
For this latest study, Prof. Howes and team conducted a review of numerous studies investigating how the primary psychoactive compound in marijuana - tetrahydrocannabinol (THC) - affects the brain.
THC exposure has long-term effects on the dopamine system
According to the researchers, there is now "substantial evidence" in animal and human studies that long-term exposure to THC leads to a decrease in levels of dopamine.
"The available evidence indicates that THC exposure produces complex, diverse and potentially long-term effects on the dopamine system," the authors explain. "These include increased nerve firing and dopamine release in response to acute THC, and dopaminergic blunting associated with long-term use."
The team believes this effect may explain why people who engage in long-term marijuana use are at increased risk for mental health problems.