Background In recent decades, cannabis has been widely used around the world for medical and recreational purposes, both legally and illegally. Aside from its therapeutic benefits, cannabis exhibits many adverse effects. Psychosis is one of the potentially harmful effects of cannabis. Case presentation A 23-year-old Thai man, who reported cannabis use for 2 years and discontinued for 3 months, restarted smoking two bongs (2 g equivalence) of cannabis. Two hours later, he had a penile erection, felt a severe persistent sharp pain in his penis, and reported that his glans looked distorted. Intending to eradicate the pain, he decided to trim the penile skin several times and completely amputated his penis himself using scissors. Cannabis-induced psychosis was diagnosed because symptoms began after cannabis use, without evidence of other substance abuse. To confirm the cannabis exposure, his urine immunoassay was positive for delta-9-tetrahydrocannabinol (Δ9-THC). The distal penis was deemed too dirty and fragile for reconstruction. Bleeding was controlled, penile stump irrigated and debrided, and scrotal urethrostomy was performed by a urologist. After admission and cannabis discontinuation, his delusion and hallucination subsided. Conclusions Cannabis-induced psychosis is an adverse effect of cannabis, which may lead to impaired judgement unexpected self-harm. A multidisciplinary team approach, including a primary care physician, an emergency physician, a urologist, and a psychiatrist, is essential when dealing with a patient with cannabis-induced psychosis and a urogenital injury.