Pharmacology
Potency versus efficacy on dose-response curves
Potency (how much drug is needed) is read from the EC50, the concentration giving half the maximum response; efficacy (how big the effect can be) from the Emax, the maximum.
Remember
A curve further to the left means greater potency, and a higher flat top (plateau) means greater efficacy, so a drug can differ in one and not the other.
A dose-response curve is a graph of the size of a drug's effect against the dose or concentration given. The dose is usually plotted on a logarithmic axis (each step along the axis multiplies the dose, for example by ten), which gives the familiar S-shaped curve, called sigmoid. Two key features of the curve are read off it.
Potency is how much drug is needed. It is measured by the EC50, the concentration at which a drug produces 50% of its own maximum response, so a lower EC50 means a more potent drug. On the graph, a more potent drug has its curve further to the left. Potency tells you the dose, not how good the response will be.
Efficacy is how big the effect can be. It is measured by the Emax, the greatest response the drug can produce, however much is given. This is the flat top (plateau) of the curve. A drug with low efficacy cannot produce a large effect, regardless of dose.
Potency and efficacy vary separately. If two drugs reach the same maximum but one works at lower concentrations, their efficacy is equal and the one further left is more potent. If one drug has the higher plateau while the other needs a lower concentration to reach half of its own maximum, the first is more efficacious and the second more potent.
Take fentanyl and morphine: fentanyl is around 100 times more potent, yet the two have similar maximum efficacy, and both are full agonists (drugs that can produce the maximum response). Paracetamol, by contrast, has much lower efficacy than morphine in severe pain.
Memory aid
left means more potent (less drug needed), whereas up means more efficacious (a higher ceiling)
Sideways shift of the curve means potency; the height of the plateau means efficacy.
Key facts
- EC50
- the concentration giving half of the maximum response; the lower it is, the more potent the drug.
- Emax
- the ceiling of the response, which extra dose does not raise, and the measure of efficacy.
- Curve further left
- less drug is needed, so the drug is more potent.
- Higher plateau
- a higher ceiling, so the drug is more efficacious.
- Independent
- a drug can be more potent without being more efficacious (having a higher ceiling), so more potent does not necessarily mean better.
- In practice
- a drug with greater potency is simply given in smaller doses.
Exam traps
- A lower EC50 describes potency, not efficacy. Efficacy is the size of the maximum response, so two drugs with the same maximum have equal efficacy, whatever their EC50 values.
- Reaching the same maximum shows equal efficacy, not equal potency. Potency depends on where the curve sits along the concentration axis, so a tenfold difference in EC50 means the potencies differ.
- A more potent drug is not necessarily a more efficacious one. Potency and efficacy vary independently, so a lower EC50 says nothing about the ceiling.
- A lower ceiling means lower efficacy, not lower potency. Lower potency shifts the curve to the right, and giving more drug can overcome it.
- Kd (the dissociation constant) tells you how tightly a drug binds, which is affinity, not the size of the response. Potency is read from the EC50 instead.
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