potency

Potency answers the question 'how little do you need?' Imagine two chili peppers that both make a dish equally spicy, but one takes a tiny pinch while the other takes a whole spoonful. The pinch-sized pepper is more potent: it does the same job with far less material. A potent drug produces its effect at a low dose or concentration.

Quantitatively, potency is read off the dose–response curve as the dose or concentration giving a chosen level of effect — most often the half-maximal point (EC50 in a tissue, or its in-vivo cousin ED50). A smaller EC50 means a more potent drug, because the curve sits farther to the left. Potency is driven largely by how tightly the molecule binds its target (its affinity) together with how efficiently binding is turned into an effect.

The crucial and constantly-misused point: potency says nothing about how big an effect a drug can ultimately reach. That ceiling is a separate property. A super-potent compound that only ever produces a weak maximal response may be far less useful than a less potent one that fully does the job. In drug discovery, chasing ever-greater potency is worthwhile only when it does not cost you the maximal effect, selectivity, or drug-like properties.