The Nature of Drugs

The Nature of Drugs
In the most general sense, a drug may be defined as any substance that brings about a change in
biologic function through its chemical actions. In the great majority of cases, the drug molecule
interacts with a specific molecule in the biologic system that plays a regulatory role. This molecule
is called a receptor. The nature of receptors is discussed more fully in Chapter 2: Drug Receptors &
Pharmacodynamics. In a very small number of cases, drugs known as chemical antagonists may
interact directly with other drugs, while a few other drugs (eg, osmotic agents) interact almost
exclusively with water molecules. Drugs may be synthesized within the body (eg, hormones) or
may be chemicals not synthesized in the body, ie, xenobiotics (from Gr xenos "stranger"). Poisons
are drugs. Toxins are usually defined as poisons of biologic origin, ie, synthesized by plants or
animals, in contrast to inorganic poisons such as lead and arsenic.
In order to interact chemically with its receptor, a drug molecule must have the appropriate size,
electrical charge, shape, and atomic composition. Furthermore, a drug is often administered at a
location distant from its intended site of action, eg, a pill given orally to relieve a headache.
Therefore, a useful drug must have the necessary properties to be transported from its site of
administration to its site of action. Finally, a practical drug should be inactivated or excreted from
the body at a reasonable rate so that its actions will be of appropriate duration.

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