The shape of the curve
Give a tissue more and more of an agonist and measure the response each time. The picture you get is the dose–response curve. At very low doses almost nothing happens; in the middle the response climbs steeply; at high doses it flattens into a plateau because the receptors are running out. On a linear axis it looks like a lopsided hill. The standard fix is to plot dose on a logarithmic axis — the log dose plot — which stretches the low end and turns that lopsided hill into a clean, symmetric S-shaped (sigmoid) curve.
This is a graded response — measured in a single tissue or person, where the effect grows smoothly with dose. (Later you'll meet its cousin, the all-or-nothing quantal response across a population, when we discuss safety.) For now, the key skill is to read two features of this S-curve separately: how high it rises, and where along the dose axis it sits.
Height is efficacy; position is potency
The height of the plateau is the Emax — the maximum effect the drug can wring out of this system. It reflects efficacy from the last guide. A full agonist reaches a tall plateau; a partial agonist tops out lower no matter how much you give.
The position of the curve along the dose axis is captured by the EC50: the concentration producing 50% of that drug's own Emax. EC50 is the headline measure of potency — how *little* drug you need to get halfway. A curve sitting far to the left (small EC50) is a potent drug; one shoved to the right needs bigger doses for the same effect.
A worked comparison
Two pain drugs, same target receptor:
EC50 Emax (max pain relief)
Drug X 2 mg 90%
Drug Y 40 mg 90%
Reading it:
- Same Emax (90%) -> same EFFICACY (equally good at best)
- Drug X EC50 lower -> MORE POTENT (needs 20x less to reach halfway)
So Drug X is more potent but NOT more effective.
A lower-potency drug given at a bigger dose can do exactly the same job.When you compare two drugs on the same log-dose plot, vertical differences are about efficacy (Emax) and horizontal differences are about potency (EC50). Train your eye to look up-and-down versus left-and-right, and the whole curve becomes readable at a glance.