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 molecule is
called a receptor. Drugs may be synthesized within the body like hormones or
may be chemicals not synthesized in the body like xenobiotics.
Poisons are also drugs
that have exclusively harmful effects. Toxins are usually defined as poisons of
biologic origin, i.e., synthesized by plants or animals. A drug is often
administered at a location distant from its intended site of action. Therefore,
a useful drug must have necessary properties to be transported from its site of
administration to its site of action.
Drugs may be solid at
room temperature (e.g, aspirin, atropine), liquid (e.g, nicotine, ethanol), or
gaseous (e.g, nitrous oxide). These factors often determine the best route of
The molecular size of
the drug varies from very small to larger one. If the molecular size of the
drug is very small it easily diffuses the cellular compartments, in case of
larger, the drug is administered directly into the vascular compartment by
intravenous or intra-arterial infusion.
Drugs interact with
receptors by means of chemical forces or bonds. There are three major types
like Covalent, Electrostatic, and Hydrophobic.
The Shape of a drug
molecule must be such as to permit binding to its receptor site via the bonds.