Drugs and their Classification

Drugs as Chemical Entities

A drug is a small organic chemical, typically in the molecular-mass range of roughly 100 to 500 atomic mass units, that is capable of interacting with a large biological macromolecule inside the body and producing a measurable physiological response. This size range matters chemically: the molecule must be small enough to be absorbed, transported and to diffuse to its site of action, yet large and structurally specific enough to fit precisely against a macromolecular binding surface.

• When the physiological response produced by a drug is beneficial and is used to diagnose, prevent or cure a disease, the substance is called a medicine.
• The same chemical, at a higher dose than the one recommended for treatment, is very often capable of acting as a poison - the difference between a medicine and a poison is frequently a matter of concentration and dose, not of chemical identity.
• The branch of chemistry concerned with using chemical substances for a therapeutic (curative) purpose is called chemotherapy.

This dose-dependence is the chemical reason doctors insist on supervised use: the same molecule that relieves a symptom at the prescribed concentration can disturb normal biochemistry, or become toxic, once that concentration is exceeded.