There is no compelling evidence that cannabis causes some psychiatric disorders in otherwise healthy individuals. Most tellingly, rates of schizophrenia and other psychiatric illnesses have remained flat even during periods when cannabis use rates increased.
For most people, symptoms of mental disturbance occur around late teens and early 20’s. Teens are more likely to experiment with cannabis and less likely to be open with parents about substance use and/or symptoms of mental disturbance. As a result, substance use/abuse has usually started by the time symptoms of mental illness become noticeable.
This is why so many studies confirm that most people diagnosed with severe mental illness have a history of alcohol and/or other substance use/abuse. The alcohol and substance use was not the cause of the mental illness, however, but a behaviour that coincides with undetected development of mental health symptoms.
Research suggests those with mental illness might actually be self-medicating with cannabis, turning to the plant to help manage symptoms, rather than becoming ill after use. Emerging evidence indicates psychiatric patients who try cannabis show significant improvements in symptoms and clinical outcomes (lower mortality rates and better cognitive functioning) compared with those who have not tried cannabis.
Some of the unique chemicals in cannabis, such as CBD, seem to have anti-psychotic properties. Researchers are investigating it as a possible treatment for schizophrenia and depression.