Pharmacodynamic Interactions

Drug Synergism:

When the therapeutic effect of two drugs are greater than the effect of individual drugs, it is said to be drug synergism.It is of two types.
(a) Additive effect: When the total pharmacological action of two or more drugs administered together is equivalent to the summation of their individual pharmacological actions is called additive effect.
i.e A + B = AB
e.g. Combination of ephedrine and aminophyllin in the treatment of bronchial asthma.
(b) Potentiation effect: When the net effect of two drugs used together is greater than the sum of individual effects, the drugs are said to have potentiation effect.
i.e AB > A + B
e.g. Trimethoprim+sulfamethoxazole

Drug Antagonism:

The phenomenon of opposing actions of two drugs on the same
physiological system is called drug antagonism.
a) Chemical antagonism: In this the biological activity of a drug can be reduced or abolished by a chemical reaction with another agent. e.g. Antagonism between acids and alkalis.
b) Competitive or reversible antagonism: In this the agonist and antagonist compete for the same receptors and the extent to which the antagonist opposes the pharmacological action of the agonist. Competitive antagonism can be overcome by increasing the concentration of the agonist at the receptor site.
e.g. Acetylcholine and atropine antagonism at muscarinic receptors.
c) Non competitive antagonism: In this type of the antagonism an antagonist inactivates the receptor (R) so that the effective complex with the agonist cannot be formed, irrespective of the agonist concentration.
e.g. Acetylcholine and papaverine on smooth muscle.
Acetyl choline and decamethonium on neuromuscular junction.
d) Physiological antagonism: When the physiological effect of a drug is antagonized by another drug by acting on two different types of receptors
e.g. Acetyl choline causes constriction where as adrenaline causes dilatation of pupil.
Importance of drug antagonism
(i) Correcting adverse effects of drugs
(ii) Treating drug poisoning.
e.g. Morphine with naloxone, organophosphate compounds with atropine.
(iii) Predicting drug combinations which would reduce drug efficacy.