Combined Effects of Drugs: Antagonism

Antagonism is an inhibitory mechanism where one drug blocks or reduces the effect of another drug. Commonly, an antagonist binds the receptor's active site and prevents agonist binding, blocking receptor activity.

Other forms of antagonism include allosteric, functional, chemical, and pharmacokinetic.

Allosteric antagonists bind receptors, such as ion channels, at a site distinct from the agonist-binding site to prevent activation.

A functional antagonist binds a different receptor to produce a physiological response opposite to the agonist-bound receptor. For example, histamine binds to histamine receptors, inducing bronchial smooth muscle contraction. In contrast, epinephrine binds β-adrenoceptors and relaxes muscle cells.

In chemical antagonism, a drug combines with another to form an inactive product. A positively charged protamine interacts with negatively charged heparin, forming an inactive salt aggregate. The aggregate prevents excessive bleeding by reversing the anticoagulant effects of heparin.

Lastly, a pharmacokinetic antagonist decreases another drug's concentration at the site of action. It does so by reducing its absorption, changing its distribution, or increasing its elimination.