Drugs can be classified according to various criteria including
chemical structure or pharmacological action. The preferred classification
is the latter one which may be divided into main groups as follows:
a) Chemotherapeutic agents - used to cure infectious diseases
and cancer. (Sulfa drugs, Antibiotics)
b) Pharmacodynamic agents - used in non-infectious diseases (Cholinergic,
Adrenergic, Hallucinogenic, Sedatives)
c) Miscellaneous agents (Narcotic Analgesics, Local Anesthetics)
Drugs have three or more names including a: chemical name,
brand or trade name, and generic or common name. The chemical
name is assigned according to rules of nomenclature of chemical
compounds. The brand name is always capitalized and is selected
by the manufacturer. The generic name refers to a common established
name irrespective of its manufacturer.
In most cases, a drug bearing a generic name is equivalent
to the same drug with a brand name. However, this equivalency
is not always true. Although drugs are chemically equivalent,
different manufacturing processes may cause differences in pharmacological
action. Several differences may be crystal size or form, isomers,
crystal hydration, purity-(type and number of impurities), vehicles,
binders, coatings, dissolution rate, and storage stability.
Introduction to Drug
A very broad definition of a drug would include "all
chemicals other than food that affect living processes."
If the affect helps the body, the drug is a medicine. However,
if a drug causes a harmful effect on the body, the drug is a
poison. The same chemical can be a medicine and a poison depending
on conditions of use and the person using it.
Another definition would be "medicinal agents used for
diagnosis, prevention, treatment of symptoms, and cure of diseases."
Contraceptives would be outside of this definition unless pregnancy
were considered a disease.
A disease is a condition of impaired health resulting from
a disturbance in the structure or function of the body. Diseases
may be classified into the following major categories:
1) Infections caused by viruses, ricketsia, bacteria,
fungi, protozoa and worms
2) Allergic diseases caused by antigens and foreign substances
3) Metabolic disorders caused by defects in the body's
ability to carry out normal reactions - these may be hereditary,
deficiency, and congenital defects
5) Toxic diseases caused by poisons
6) Psychosomatic and mental diseases
Chemotherapy, broadly defined, means the treatment of any
disease by chemicals including infectious and non-infectious
diseases. The original definition applied only to drugs which
were used in the treatment of infectious diseases. The proper
term for the treatment of non-infectious diseases is pharmacodynamics.
Sites of Drug Action:
l. Enzyme Inhibition:
Drugs act within the cell by modifying normal biochemical
reactions. Enzyme inhibition may be reversible or non reversible;
competitive or non-competitive. Antimetabolites may be used which
mimic natural metabolites. Gene functions may be suppressed.
2. Drug-Receptor Interaction:
Drugs act on the cell membrane by physical and/or chemical
interactions. This is usually through specific drug receptor
sites known to be located on the membrane. A receptor is the
specific chemical constituents of the cell with which a drug
interacts to produce its pharmacological effects. Some receptor
sites have been identified with specific parts of proteins and
nucleic acids. In most cases, the chemical nature of the receptor
site remains obscure.
3. Non-specific Interactions:
Drugs act exclusively by physical means outside of cells.
These sites include external surfaces of skin and gastrointestinal
tract. Drugs also act outside of cell membranes by chemical interactions.
Neutralization ofstomach acid by antacids is a good example.
Mode of Drug Action:
It is important to distinguish between actions of drugs and
their effects. Actions of drugs are the biochemical physiological
mechanisms by which the chemical produces a response in living
organisms. The effect is the observable consequence of a drug
action. For example, the action of penicillin is to interfere
with cell wall synthesis in bacteria and the effect is the death
of the bacteria.
One major problem of pharmacology is that no drug produces
a single effect. The primary effect is the desired therapeutic
effect. Secondary effects are all other effects beside the desired
effect which may be either beneficial or harmful. Drugs are
chosen to exploit differences between normal metabolic processes
and any abnormalities which may be present. Since the differences
may not be very great, drugs may be nonspecific in action and
alter normal functions as well as the undesirable ones. This
leads to undesirable side effects.
The biological effects observed after a drug has been administered
are the result of an interaction between that chemical and some
part of the organism. Mechanisms of drug action can be viewed
from different perspectives, namely, the site of action and the
general nature of the drug-cell interaction.
l. Killing Foreign Organisms:
Chemotherapeutic agents act by killing or weakening foreign
organisms such as bacteria, worms, viruses. The main principle
of action is selective toxicity, i.e. the drug must be more toxic
to the parasite than to the host.
2. Stimulation and Depression:
Drugs act by stimulating or depressing normal physiological
functions. Stimulation increases the rate of activity while depression
reduces the rate of activity.