Emax

Emax is the height of the plateau on a dose–response curve — the biggest effect a drug can ever produce, no matter how much more you give. Picture filling a glass under a tap: once the glass is full, opening the tap wider does not add more water. Emax is that 'full glass' level for a drug's response.

On the standard sigmoidal dose–response model, the response climbs with dose and then levels off at Emax. The dose sitting at half of Emax is the EC50, so the two numbers describe different things: EC50 (or its log, related to potency) tells you where the curve sits left-to-right, while Emax tells you how high it goes. A higher-efficacy drug has a higher Emax; a partial agonist's Emax falls short of what a full agonist can reach in the same system.

Emax is a property of the drug-and-system together, not the drug alone. The same molecule can show a higher Emax in a tissue rich in receptors (or with abundant spare receptors) and a lower Emax where receptors are scarce. So when comparing two compounds, the comparison is only fair within the same assay or tissue under the same conditions.