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Potency and the Dose–Response Curve

How do we put a number on "how much drug do you need"? Meet the dose–response curve, the EC50 and IC50 that summarize it, and the careful distinction between potency and efficacy that defines a drug's profile.

The curve everything hangs on

Give a tiny dose of an agonist and you see almost no response. Give a huge dose and the response plateaus — adding more does nothing. Plot response against dose and you get the dose–response curve, an S-shape (sigmoid) when dose is drawn on a log scale. Almost every quantity in pharmacodynamics is read off this one picture.

Two features of the curve carry the meaning. Its height — the plateau — is the maximum effect the drug can reach, the Emax. Its horizontal position — how far left or right it sits — tells you how much drug you need. A curve sitting to the left means a small dose already works; a curve to the right means you need much more. We summarize that position with a single concentration.

EC50, IC50, and the meaning of potency

The standard summary of position is the concentration giving half of the maximum effect. For an activator we call it the EC50 (half-maximal effective concentration); for an inhibitor, the IC50 (half-maximal inhibitory concentration). A lower EC50 or IC50 means the curve sits further left — less drug is needed. That is exactly what we mean by potency: how much drug you need to get a given effect. A nanomolar IC50 drug is more potent than a micromolar one because it does the same job at a thousandth of the concentration.

Two cough drugs, same target:

  Drug X   IC50 = 5 nM    Emax = 90% of cough suppressed
  Drug Y   IC50 = 500 nM  Emax = 90% of cough suppressed

  -> Same EFFICACY (both reach 90%).
  -> Drug X is 100x more POTENT (works at 1/100th the dose).

Now a different pair:

  Drug P   IC50 = 5 nM    Emax = 95%
  Drug Q   IC50 = 5 nM    Emax = 40%

  -> Same POTENCY (same IC50).
  -> Drug P has higher EFFICACY (reaches a higher ceiling).
Potency (IC50/EC50) and efficacy (Emax) move independently — read them off different axes.

Why potency is not the whole story

Beginners often chase potency as if smaller IC50 were the only goal. It is not. A super-potent compound that cannot dissolve, cannot cross membranes, or hits five other targets is worse than a modestly potent one that behaves. Potency buys you a lower dose; it does not buy safety, selectivity, or a clean curve. Keep potency in its lane: it is the x-axis position of one curve, measured under one set of conditions.