Marijuana use, especially at a young age, may increase the risk of developing schizophrenia or other mental illnesses. The risk is highest for teenagers who start smoking marijuana before age 16, as they are four times more likely to become schizophrenic. Genetics also play a role, as variations in the COMT gene can double or quintuple someone's risk depending on which variants they carry. The document profiles three individuals who developed serious mental health issues like bipolar disorder or schizophrenia after starting marijuana use as teenagers.