competitive antagonism

Competitive antagonism is a tug-of-war for the same seat. The antagonist and the agonist both reach for the identical binding site, so whoever is more abundant tends to win — and you can always tip the balance back by flooding the system with more agonist.

Because the antagonist binds reversibly to the agonist's own site, the blockade is surmountable: enough agonist will eventually out-compete it and still reach the full maximal effect. On a dose–response curve this shows as a parallel rightward shift — the agonist needs a higher concentration (a larger EC50, appearing less potent) but its Emax is unchanged.

This reversibility is clinically valuable: the depth of block depends on the ratio of the two drugs, so the effect can be adjusted or reversed. Naloxone reversing opioids and atropine antagonising acetylcholine at muscarinic receptors are classic examples. The picture differs from non-competitive antagonism, where raising the agonist dose cannot restore the full response.