Two questions, two answers
Look again at a log dose–response curve. It has a horizontal position (how far left or right it sits) and a height (how high its plateau reaches). These two features answer two different questions, and they are independent.
Potency is the horizontal position: how much drug you need to reach a given effect. We summarise it with the EC50 (the concentration giving half of Emax) or the ED50. A curve farther to the left is more potent — it does the job at a lower dose. Efficacy is the height of the plateau: the largest effect the drug can ever produce, its Emax. A curve that reaches higher has greater efficacy, no matter where it sits left to right.
Why efficacy almost always wins
Patients feel efficacy, not potency. Compare two painkillers: morphine reaches a far higher Emax than ibuprofen, so for severe pain it can do things ibuprofen simply cannot, regardless of dose. That is an efficacy difference, and it is decisive. Meanwhile, if one drug needs 5 mg and another needs 500 mg for the same maximum relief, the only practical consequence of that potency gap is the size of the pill. We can almost always make a less potent drug work by giving more of it — but we can never push a low-efficacy drug past its ceiling.
Two drugs, same target, on a log-dose plot:
Effect
Emax_A | ____________ Drug A (high efficacy)
| /
1/2 A |- - - x- - - - - - - -
| / ____________ Drug B (lower efficacy, ceiling here)
| / ___/
0 |___/__/______________________ log dose -->
^ ^
more less
potent potent
Left-right shift = potency difference
Plateau height = efficacy difference (Drug A's ceiling is higher)Where this bites in practice
Marketing and casual conversation constantly blur these. A pill advertised as 'the strongest' usually just means more milligrams per tablet — a potency claim dressed up as a benefit. Within a drug class, potency mainly tells you the number to write on the prescription, while efficacy tells you whether the drug can reach the result you need at all. Knowing which is which keeps you from over-valuing a small number on a label.