Drug interactions may be defined as an
alteration in duration and/or onset of action
of the pharmacokinetic and/or pharmaco-
dynamic of one drug produced by another
drug. The multiple drug therapy produced
a combined effect, which may be antagonis-
tic or synergistic in nature. In antagonism
the effects of one drug are reduced or abol-
ished by the second drug, and in synergism
the effects may be additive or potentiative
in nature, which may be harmful or useful
to the patient in a particular disease.
The clinically established drug
interactions can be minimized to some
extent by the avoidance of combined drug
therapy, which are proven to be
incompatible.
But, in certain cases, the single drug is
effective only to a certain degree or stage of
disease condition. The multiple/combined
drug therapy is required in many medical
and dental conditions.
- To produce a desired pharmacody-
namic/ therapeutic effect which is not
obtained by single drug, e.g. in the
treatment of hypertension, a single
drug is effective only in very low per-
centage of patients. Likewise, in the
treatment of heart failure, a combined
therapy of diuretic with vasodilator
and/or cardiac glycoside has to be
given to achieve a adequate cardiac
output and control over edema. Mul-
tiple drug therapy also required in che-
motherapy of cancer, tuberculosis and
certain infectious diseases.- To minimize the side effects of drugs
e.g. to supplement potassium by giving
potassium sparing diuretic with
digitalis.
The use of combined drug therapy can
not be avoided in certain cases to attain a
desired therapeutic level, but the risk of
incompatibility/interactions involved in the
treatment increases.
The drug interactions may be divided
into:
i. Pharmacokinetic, which occur at the
level of absorption, distribution,
metabolism and excretion of one drug
by another.
ii. Pharmacodynamic, which occur at the site
of drug action involving the receptors.