drug
A drug is a chemical you take to change how your body works, usually to fight a disease or relieve a symptom. Think of it as a small, precisely shaped key that you swallow or inject so it can fit into one specific lock inside the body and switch some biological process on or off.
More formally, a drug is a chemical substance that produces a therapeutic or biological effect when administered to a living organism. It works by interacting with a particular target molecule, such as a protein, and altering that target's activity in a way that benefits the patient.
The word covers a wide range, from a single small organic molecule like aspirin to large proteins like insulin. Importantly, the same substance can heal at one dose and harm at another, so the difference between a medicine and a poison is often just the amount given.
Aspirin is a drug that blocks the enzyme cyclooxygenase, reducing the inflammatory signals that cause pain and fever.
A familiar example of a chemical that produces a defined therapeutic effect.
In everyday speech 'drug' can mean a substance of abuse; in medicinal chemistry it neutrally means any therapeutic agent.