Efficacy

The term efficacy relates to the pharmacodynamic effectiveness of drugs. However, the terms efficacy and effectiveness are not exactly interchangeable. Efficacy is the maximal effect of an agonist (a drug or chemical that activates a specific receptor system). This occurs when the dose is taken at very high levels. The efficacy is determined primarily by the receptor system (the molecules in a biological system that interact with drugs to change the functioning of the system) and by the effector system (effectors are the molecules that translate the drug-receptor interaction into an actual change in the activities of the cell). Efficacy can be measured with a graded dose–response curve. A graded dose–response curve is a graphical representation of the response of a specific receptor–effector system measured against an increasing concentration of a specific drug. Traditionally, it is graphically represented as a sigmoid curve on semilogarithmic axes where efficacy is depicted by the plateau of the dose–response curve. The efficacy (maximum effectiveness) and potency (the dose causing 50% of the maximum effect) parameters can be derived from these data.

It is important to recognize that undesirable effects (side-effects) may limit the dose of a drug so that the concentration associated with its maximal desirable effect is not achieved. The therapeutic index of a drug represents the margin of safety. It is the difference between the dose of the drug that produces a desired effect and the dose that produces undesirable effects. The therapeutic index of a drug is often defined as the ratio between the median lethal dose and the median effective dose (LD50/ED50).