Potency vs Efficacy
Potency is the concentration a drug needs to produce a given effect, while efficacy is the largest effect it can produce at any concentration; the two are independent properties.
Potency and efficacy describe different axes of the same dose-response curve. Potency is horizontal position — the concentration required, summarised as EC50 or IC50 — so a compound acting at 1 nanomolar is a hundred times more potent than one acting at 100. Efficacy is vertical height, the Emax reached once every receptor it can occupy is occupied. A drug can be extraordinarily potent and barely efficacious, or weakly potent and able to drive the system to its ceiling.
Potency alone determines milligrams. Semaglutide is given at 2.4 mg once weekly for weight management while liraglutide is given at 3.0 mg once daily, a gap reflecting both potency and half-life; neither figure predicts how much weight either produces. Efficacy is what head-to-head trials measure. Likewise a partial agonist can bind at picomolar concentrations and still plateau at half the response of a less potent full agonist.
Which quantity you need depends on the question. Potency governs how much material a dose contains and, indirectly, the room between a therapeutic concentration and an off-target one — a compound hitting its target at 1 nanomolar and a secondary receptor at 1 micromolar has a workable window. Efficacy governs the best case: no dose increase raises a ceiling set by intrinsic efficacy.
The word potent does the damage. In marketing it means strong, effective or better, none of which it means pharmacologically. Spec sheets compound this by quoting EC50 values from different assay systems as if comparable, when receptor overexpression, readout choice and cell line each shift the number by an order of magnitude. A potency figure is never an efficacy claim.