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Dose-Effect Curve:
Dose-Effect Curve:
A characteristic, even the sine qua non, of a true drug effect is that a larger dose produces a greater effect than does a smaller dose, up to the limit to which the cells affected can respond. While characteristic of a drug effect, this relationship is not unique to active drugs, since increasing doses of placebos (q.v.) can, under certain conditions, result in increasing effects. Distinguishing between " true" and "inactive" drugs requires more than demonstration of a relationship between "dose" and effect.
The curve relating effect (as the dependent variable) to dose (as the independent variable) for a drug-cell system is the "dose-effect curve" for the system. For a unique system, i.e., one involving a single drug and a single effect, such curves have three characteristics, regardless of whether effects are measured as continuous (measurement) or discontinuous (quantal, all-or-none) variates:
1. The curves are continuous, i.e. there are no gaps in the curve, and effect is a continuous function of dose. Some effect corresponds to every dose above the threshold dose (q.v.), and every dose has a corresponding effect; there is no inherent invalidity in interpolating doses or effects from a dose-effect curve.
2. The curves are "monotonic". The curve may have a positive slope, or a negative slope, but not both if the system under study is unique. The slope of the curve may show varying degrees of positivity (negativity), but the sign of the slope stays the same throughout the range of testable doses. When monotonicity of a dose-effect curve does not obtain, one may infer that the system under study is not unique or singular: either more than one active agent or more than one effect is under study.
3. The curves approach some maximum value as an asymptote, and the asymptote is a measure of the intrinsic activity (q.v.) of the drug in the system.
Cf. Bioassay, Median Effective Dose, Time-Concentration Curve, Dose-Duration Curve, Metameter
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