It’s complicated. This WBUR article does a really fabulous job of looking at facts and not sensationalizing an oft-sensationalized topic. I really do believe that cannabis increases the risk of psychosis for some people. My former colleague at McLean Hospital, Dr. Dost Öngür, chief of the Psychotic Disorders Division, makes a compelling case to hold off using cannabis until the age of 30.
Leading Psychosis Expert To His Students: To Avoid Risk, Hold Off On Pot Til 30
Front Range CBT - Psychotherapy for Adolescents and Adults 