What is a Drug?

In casual conversations, social media and news media, the word “drug” is usually used with strong negative undertones, referring to substances that may be addictive, unsafe or illegal (eg cocaine, heroin, cannabis). In understanding how substances affect the body, cultural/legal ickiness is low in importance. Therefore, pharmacologists choose to use the term "drug" in a more neutral manner.

In pharmacology, we use the term “drug” broadly to refer to any chemical which alters the physiology of an organism. You'll note that when pharmacologists use the term “drug”, they don't make any distinctions about beneficial vs harmful, therapeutic vs recreational, natural vs synthetic, or legal vs illegal. This is widely accepted standard terminology in pharmacology but. However, I find people can be quite resistant to this framing of the term “drug” so I’ve compiled a list of definitions coming from some of the most influential pharmacology textbooks below.

  • Rang and Dale’s Pharmacology (7e): ...a chemical substance of known structure, other than a nutrient or an essential dietary ingredient, which, when administered to a living organism, produces a biological effect
  • Goodman and Gilman’s The Pharmacological Basis of Therapeutics (9e): ...any chemical agent that affects processes of living
  • Katzung’s Basic & Clinical Pharmacology (13e): ...any substance that brings about a change in biologic function through its chemical actions
  • Tripathi Essentials of Medical Pharmacology (7e): ...any single chemical substance which can produce a biological response

The word “drug” probably derives from an old Dutch term “droge”, referring to barrels of dried plants (early medicines).

Drugs vs Non‑Drugs

Since pharmacology uses such a broad definition of drugs, it will be helpful to look at grey areas to clarify which substances should be included in this definition and which substances should be excluded.

Nutrients

Nutrients (eg carbohydrates, fats, protein, vitamins, minerals) are substances required for normal physiological functions. It's debated whether nutrients can be considered drugs when given to correct a deficiency (eg vitamin C given to treat scurvy). In such scenarios, the administered substance—the nutrient—does not alter physiology. They cannot be considered drugs because they do not alter the physiology of the organism. On the contrary, they define normal physiology.

An exception is when nutrients are given in supraphysiological doses to alter physiology beyond normal nutritional needs. For example, vitamin B6 is given at 50-100x the normal dietary doses to help manage nausea during pregnancy. In this scenario, I think it is fair to say the nutrient is being used as a drug.

Poisons

“Poison” is not a pharmacological category. A poison is simply a drug used at a harmful dose or in a harmful context. The same substance which is life‑saving in one context can be life-ending in another. Most drugs can also be poisons so pharmacologists make no distinction.

Endogenous Substances

Hormones and neurotransmitters produced inside the body are not usually considered drugs. However, when the same substances are administered from outside the body (ie exogenously), they are considered drugs. Examples include insulin, adrenaline and testosterone.

Natural Products

Some plant products have very clear physiological effects:

  • Coffee from Coffea species—stimulant
  • Wine from Vitis vinifera—intoxicant
  • Opium from Papaver somniferum—pain reliever
  • Tobacco from Nicotiana species—stimulant

Although these plant products have well-defined, reproducible physiological effects, they are not drugs per se. Instead, the active compound(s) in these plant products—caffeine, ethanol, morphine and nicotine respectively—that are responsible for these effects are drugs.

Sources of Drugs

Drugs can come from a wide variety of sources. The source gives no additional information about whether a drug is good, bad, safe, or unsafe. Those properties depend on dose, context, and pharmacology.

Natural Drugs

Many drugs originate from plants, microbes, or animals. Examples include morphine (from poppy), penicillin (from fungus), and heparin (from animal tissues).

Semi‑Synthetic Drugs

Semi-synthetic drugs begin as natural molecules which are chemically modified to improve their potency, pharmacokinetic properties, side effect profile or aid manufacturing. Examples include oxycodone (derived from morphine), amoxicillin (derived from penicillin) and enoxaparin (derived from heparin).

Fully Synthetic Drugs

These are designed and built entirely "from scratch" using medicinal chemistry techniques to target a specific receptor/enzyme in the body. Many modern medicines fall into this category. Fully synthetic drugs may draw on insights from natural products or endogenous ligands of the biological target.

A Plea for Destigmatisation

Because the word “drug” carries heavy social and emotional baggage, students often react negatively when they first encounter the pharmacological definition. But destigmatising and using neutral terminology helps for scientific clarity, effective healthcare, and progress in research.

Neutral terminology allows pharmacologists to compare substances objectively without moral judgement. It lets us study heroin, penicillin, caffeine, cocaine, insulin and ibuprofen using the same unbiased scientific framework.

For our patients, stigma around the word “drug” can lead to:

  • Patients avoiding beneficial medicines (eg fentanyl for severe pain).
  • Patients preferring "natural" alternatives to a more effective drug.
  • Unforeseen drug interactions in patients who use illegal drugs.

Stigma also affects progress in scientific and clinical research. Research on some drug classes has been severely restricted in the past due to cultural attitudes rather than scientific concerns. This has slowed the development of potentially beneficial therapies, including:

  • Psychedelics for refractory mental health conditions.
  • Cannabinoids for epilepsy and chronic pain.
  • Opioids with reduced addiction potential.

Quiz