Key Terminologies Related to Substance Use Disorders Addiction – A chronic, relapsing disorder characterized by compulsive drug-seeking, continued use despite harmful consequences, and long-lasting changes in the brain. Gateway Drug – A substance (e.g., nicotine, alcohol, cannabis) that is believed to lead to the use of more addictive or dangerous drugs. Substance-Induced Disorder – A condition where substance use leads to psychiatric symptoms such as depression, anxiety, or psychosis. Cross-Tolerance