Emax

Emax is the ceiling of a drug's effect — the most it can ever do, no matter how much more you give. Once you reach Emax, adding drug only spends molecules without buying any extra benefit.

On a dose–response curve, Emax is the height of the plateau where the line flattens out. It is the experimental expression of a drug's efficacy: full agonists reach a high Emax, partial agonists plateau at a lower Emax, and pure antagonists alone produce no effect at all. The plateau usually appears because the response saturates downstream — for instance every receptor or every coupling step is already maximally engaged.

Emax is conceptually separate from potency and EC50, which describe where on the dose axis the effect happens. Knowing the Emax matters clinically because once a drug has reached its ceiling, pushing the dose higher adds only side effects and toxicity, not more therapeutic benefit.