With cannabis potency at historic highs, this study underscores that adolescent brains are uniquely vulnerable to THC exposure, and parents should understand the psychiatric risks before dismissing cannabis as harmless. Data from a JAMA Health Forum study of nearly half a million teenagers demonstrates that adolescent cannabis use doubles the risk of psychotic and bipolar disorder diagnoses by early adulthood. The association persisted across demographic subgroups and was temporally consistent, with cannabis use preceding psychiatric diagnoses by roughly two years on average.