A therapist friend recently attended a conference where a big group of doctors and health professionals focused around adolescents presented on the link between pot and the development of psychosis and even schizophrenia in users under age 32 or something like that. I've definitely heard about this colloquially with people reacting very differently to the same strain ("we were all fine but she was just freaking out")…
Every study done, and likely to ever be done given ethical standards, attempting to link marijuana usage to mental illness shares the same fundamental problem which makes their results meaningless: It cannot account for self-selection. There is no way to know that those who would later be diagnosed with schizophrenia were not self medicating with marijuana, leading to the causation actually being backwards. And in th…
Ignoring the link between mental illness and marijuana use is ridiculous. A medical study doesn't need 100% confidence to be true. If we held all of medical science to that bar, we'd never make progress.
Plus, we have a colloquialism for a person whose mind appears to have suffered damage from excessive marijuana usage: a burnout. It's something that even recreational users observe. And where there's smoke...