EC50

The EC50 of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed.

Biological responses to ligand concentrations typically follow a sigmoidal function.

While relying on a graph for estimation is more convenient, this typical method yields less accurate and precise results.

E increases with [A] until at sufficiently high [A], when E plateaus towards an asymptotic maximum attainable response, Emax.

The lower the EC50, the less the concentration of a drug is required to produce 50% of maximum effect and the higher the potency.

Faster or stronger binding is represented by a higher affinity, or equivalently a lower dissociation constant.

Efficacy is the relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level.

The exact shape of this time function will depend upon the stressor (e.g., the specific toxicant), its mechanism of action, the organism exposed, etc.

[citation needed] Furthermore, EC50 is dependent on many factors including species, tissue and cell type and genetics.

The tissue response (y-axis) to an agonist, in log concentration (x-axis), in the presence of different antagonist concentrations. The EC 50 of the agonist is represented by the x co-ordinate that corresponds with the half-maximum of the leftmost curve. This is denoted by [A]