A drug is any substance that, when taken into the body, alters its structure or function. Drugs can be prescribed for medical purposes, used recreationally, or abused.
Addiction is a psychological and physiological dependence on a substance or behavior. It involves compulsive drug-seeking and use despite negative consequences.
Pharmacology: Pharmacology is the study of how drugs interact with biological systems, including their effects, mechanisms of action, and therapeutic uses.