What is the definition of a drug?

A drug is defined as a chemical substance that alters one's behavior or feelings. This definition encompasses a wide range of substances, including both legal and illegal drugs, that can impact the central nervous system and affect mood, perception, and consciousness. Drugs can lead to various physical, emotional, and psychological effects, which is fundamental to understanding their use, whether in medical contexts or in discussions about substance abuse.

The other options either focus too narrowly on specific aspects of drugs or introduce concepts unrelated to the fundamental definition. Legal policies are about regulations and do not define what a drug is, while categorizing medications for mental health or illegal substances focuses on specific types of drugs rather than capturing the broader definition that encompasses all substances affecting behavior and feelings.