Bryan McAllister’s personal reference notebook
September 11, 2026Short & citedBlog
Civic Viewpoint.
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Kept by Bryan McAllister — short guides, clear facts, and the Canadian angle when it matters.
Civic Brief • Substance

Drug

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

Updated September 11, 20261 min readCited sources

A drug is a chemical substance that produces a biological effect on the body or mind. The term covers medications, psychoactive substances, and illegal drugs depending on context.

What it is

A drug is any chemical substance that, when administered to a living organism, alters normal body or brain function. In science and medicine, the definition is broad and includes everything from prescription medicines to recreational substances.

In healthcare, drugs are used to diagnose, treat, prevent, or manage disease. Legal classification varies by jurisdiction: some drugs are available over the counter, others require a prescription, and many are controlled or illegal under federal and provincial laws.

Risks and misuse

Misuse of drugs — taking them in ways not intended — can lead to tolerance, dependence, withdrawal, and addiction. Psychoactive drugs that affect the central nervous system carry particular risks when used repeatedly or non-medically.

In short

  • A drug is any chemical that affects a living organism.
  • In medicine, drugs include prescription and over-the-counter products.
  • Psychoactive drugs alter mood, perception, or consciousness.
  • Misuse can lead to dependence and addiction.
Canadian angle

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.

Quick 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.
What is a psychoactive drug?
A psychoactive drug affects the central nervous system and can alter mood, perception, consciousness, or behaviour.

Sources

  1. MedlinePlus — https://medlineplus.gov/druginformation.html
    Supports: General drug and medicine information, including prescription and over-the-counter medicines.
  2. 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.
  3. MedlinePlus — https://medlineplus.gov/ency/patientinstructions/000798.htm
    Supports: Prescription drug misuse, addiction, tolerance, and withdrawal.
  4. 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.