Pharmacology for Dentistry

44 Section 1/ General Principles of Pharmacology

synergistic. In the synergism, the drugs can
have action in the same direction or when
given alone, one may be inactive. Synergism
can be additive or supradditive in nature.

ADDITIVE

When the effect of two drugs are in the
same direction. For example when aspirin
is combined with paracetamol the combined
effect is analgesic/antipyretic.

Another important example is
combination of theophylline and ephedrine
as bronchodilator, combination of
sulfonamides as antibacterial etc.

SUPRADDITIVE

In this the effect of combined therapy is
greater than the individual effect of the one
drug. Examples are:

• Levodopa and peripheral dopa-
  decarboxylase inhibitor, carbidopa or
  benserazide in the treatment of
  parkinsonism.


• Sulfonamide and trimethoprim (well
  known preparation ‘Septran’) as
  antibacterial.

DRUG ANTAGONISM

Antagonism describes the situation, when
one drug decreases or inhibits the action
of another. The antagonism may be physi-
cal in which the physical property of the
drug can affect the absorption of another
drug, chemical in which two drugs react
chemically and form a biologically inactive
compound. This type of reaction may be
used in the treatment of drug poisoning.
For example in cyanide poisoning nitrites
form methaemoglobin which has high af-
finity for cyanide radical and forms

cyanomethaemoglobin and after sodium

thiosulphate injection forms sodium thio-

cyanate which is easily excreted in urine.

On receptor basis, drug antagonism will

be of two types competitive and noncompeti-

tive antagonism.

COMPETITIVE ANTAGONISM

In competitive antagonism, the

antagonist binds with the same receptor as

agonist. If the log dose response curve with

agonist is obtained in the presence of

antagonist, it will be found that antagonist

has no effect of its own and there is parallel

rightward shift in the dose response curve

of agonist (Fig. 1.5.2) with no change in

shape, slope or maximum response.

In competitive antagonism, the

antagonist reduces affinity i.e. potency of the

agonist.

Example of competitive antagonism are:

• Acetylcholine (as agonist) – atropine (as
  antagonist).


• Acetylcholine – d-tubocurarine.


• Isoprenaline – propranolol.

Fig. 1.5.2: Log dose response curve showing

competitive antagonism.