PharmacologyTier 1Medical Sciences concept

Pharmacodynamic terminology - potency vs efficacy

Core concept

A graded dose-response curve has two independent dimensions, and they are routinely confused

	Definition	Curve parameter	Curve feature
Potency	The concentration needed to produce a given effect	EC50 (or ED50)	Horizontal position - left = more potent
Efficacy	The maximum effect achievable	Emax	Height of the plateau

Two drugs can differ in potency but share efficacy (all loop diuretics), or share potency but differ in efficacy (a partial vs full agonist)
Potency is largely irrelevant clinically - it is corrected by changing the dose. Efficacy cannot be corrected by dosing

Affinity - how tightly a drug binds the receptor (Kd); determines potency
Intrinsic activity - the response produced once bound; determines efficacy
- Full agonist = 1, partial agonist = between 0 and 1, antagonist = 0, inverse agonist < 0
A partial agonist has a lower Emax no matter the dose, and acts as an antagonist in the presence of a full agonist
- Buprenorphine - partial mu agonist; ceiling on respiratory depression (safety) but precipitates withdrawal in someone on a full agonist
- Varenicline, aripiprazole (dopamine partial agonist), pindolol (beta partial agonist)

Antagonism

Type	Effect on the curve
Competitive, reversible	Rightward shift, Emax preserved - surmountable by more agonist
Non-competitive / irreversible	dec Emax, curve depressed - not surmountable
Allosteric	Modulates affinity or efficacy, positive or negative

Naloxone is competitive -> a large enough opioid dose overcomes it (why bigger naloxone doses are needed for buprenorphine or high-potency fentanyl analogues)
Aspirin is irreversible -> the effect persists for the platelet lifespan regardless of concentration
Spare receptors - maximum response reached with less than full occupancy; EC50 then lies left of Kd
Examples worth carrying
- Frusemide is less potent but more efficacious than bendroflumethiazide - the ceiling is what matters in heart failure
- Fentanyl is ~100x more potent than morphine, with the same efficacy
- Rosuvastatin is more potent than pravastatin; both are full statins

"Stronger" almost always means efficacy, not potency - the clinically important question is whether the drug has a higher ceiling
- Escalating a thiazide in advanced CKD is futile; switching to a loop diuretic is not - a ceiling problem, not a dose problem
Partial agonism explains buprenorphine's safety profile and its induction problem, varenicline's dual action, and why aripiprazole is less prone to hyperprolactinaemia
Understanding competitive vs irreversible antagonism explains why naloxone must be re-dosed, and why aspirin cannot be
Potency comparisons drive equivalent-dose tables (opioids, corticosteroids, benzodiazepines) - the exam tests these conversions

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