Does cannabis cause mental illness?

Cannabis use is associated with increased risk of psychosis, schizophrenia, depression, and anxiety disorders, but the causal relationship is complex. For psychosis: the evidence is strongest. A 2019 Lancet Psychiatry study found daily high-potency cannabis use was associated with a 5-fold increased risk of psychosis. The risk is dose-dependent, potency-dependent, and substantially higher in those with genetic predisposition (COMT Val158Met polymorphism). For schizophrenia: cannabis use is associated with earlier onset and worse outcomes, but whether it causes schizophrenia in those without predisposition remains debated. For depression and anxiety: associations exist but causality is unclear — people with depression and anxiety may self-medicate with cannabis. The adolescent brain appears particularly vulnerable; cannabis use before age 16 is associated with substantially higher mental health risks than adult-onset use.