A drug can be defined as a chemical substance of a known structure, other than a nutrient or essential dietary ingredient, which brings about a change in biological function chemical reactions. A drug is quite different from a medicine. A medicine is a chemical preparation, which usually, but not necessarily, contains one or more drugs, administered with the intention of producing a therapeutic effect. Medicines usually contain other substances (excipients, stabilisers, solvents, etc.) besides the active drug, to make them more convenient to use. To count as a drug, the substance must be administered as such, rather than released by physiological mechanisms.
Many substances such as insulin or thyroxine, are endogenous hormones but are also drugs when they are administered intentionally. In most cases, the drug molecule interacts as an agonist (activator) or antagonist (inhibitor) with a specific molecule in the biologic system that plays a regulatory role. This target molecule is called a receptor.
DRUG RECEPTORS
The term receptors describe protein molecules whose function is to recognise and respond to endogenous chemical signals. Other macromolecules with which drugs interact to produce their effects are know as drug targets. They also includes enzymes, carrier molecules (transporters) and ion channels.
DRUG SPECIFICITY
For a drug to be useful as either a therapeutic or a scientific tool, it must act selectively on particular cells and tissues. In other words, it most show a degree of binding site specificity. Conversely, proteins that function as drug targets generally show a high degree of ligand specificity; they bind only molecules of a certain precise type.
Specificity is reciprocal: individual classe of drugs bind only to certain targets, and individual targets recognise only certain classes of drug. No drugs are completely specific in their actions. In most cases, increasing the dose of a drug will cause it to affect targets other than the principal one, and this leads to side effects.
Ref: Rang & Dale's Pharmacology
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