Crime & Justice • Substance

Drug

A drug is a chemical substance that has a biological effect on the body or mind when taken or administered.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

A drug is a chemical substance that has a biological effect on the body or mind when taken or administered. In medicine, the term commonly refers to a medication used to diagnose, treat, prevent, or relieve disease; in other contexts, it can also refer to psychoactive or illegal substances. The meaning depends on context and legal classification.

Definition

The word drug has multiple meanings, but in science and medicine it generally means any chemical substance that produces a biological effect in an organism. In everyday use, it may also refer more narrowly to a medicine or to a psychoactive substance.

Medical use

In healthcare, drugs are used to diagnose, treat, prevent, or manage diseases and symptoms. This includes prescription medicines, over-the-counter products, and some biologically active substances used in hospitals and clinics.

Psychoactive and recreational use

Some drugs act on the brain and central nervous system, changing mood, perception, alertness, or consciousness. These may be used legally, illicitly, or non-medically, depending on the substance and jurisdiction.

Misuse and dependence

Drug misuse can cause tolerance, meaning more is needed for the same effect, and dependence, where stopping causes withdrawal symptoms. Public health sources also distinguish prescribed use from non-medical use and drug use disorder.

Key facts

A drug is typically defined by its biological effect on a living organism.
In medicine, drugs include prescription medicines and many over-the-counter products.
Psychoactive drugs affect the central nervous system and can alter mood, perception, or consciousness.
Misuse of drugs can lead to tolerance, dependence, withdrawal, and addiction.
Legal definitions of drug vary by country and regulatory system.

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Canadian context

In Canada, drugs are regulated under federal and provincial health and controlled-substances laws, with separate rules for prescription medicines, over-the-counter products, and controlled substances.

Frequently asked questions

What is a drug?

A drug is a chemical substance that produces a biological effect in the body or mind when administered to a living organism.

Is a medication a drug?

Yes. In medical contexts, a medication or medicine is a type of drug used to diagnose, treat, prevent, or relieve disease.

Are all drugs illegal?

No. Many drugs are legal and widely used as medicines, while others are controlled or illegal depending on the substance and jurisdiction.

What is a psychoactive drug?

A psychoactive drug affects the central nervous system and can alter mood, perception, consciousness, or behaviour.

What is drug misuse?

Drug misuse is using a medicine or other drug in a way not intended, such as taking it for non-medical effects or not as prescribed.

Can drugs cause addiction?

Some drugs can lead to addiction, dependence, and withdrawal, especially when used repeatedly or in ways that change brain and body responses.

References

MedlinePlus — https://medlineplus.gov/druginformation.html
Supports: General drug and medicine information, including prescription and over-the-counter medicines.
National Institute on Drug Abuse — https://nida.nih.gov/research-topics/drugs-a-to-z
Supports: Drug classes, opioid examples, and effects on the body and brain.
MedlinePlus — https://medlineplus.gov/ency/patientinstructions/000798.htm
Supports: Prescription drug misuse, addiction, tolerance, and withdrawal.
ENCYCLOPEDIA of DRUGS, ALCOHOL & ADDICTIVE BEHAVIOR — https://www.cur.ac.rw/mis/main/library/documents/book_file/digital-6405faeae0b7b2.08636337.pdf
Supports: General encyclopedic context for drugs, alcohol, and addictive behavior.