The shape of the Cp/response curve is determined by pharmacodynamic factors. Cp/response curves reflect the result in a number of individuals, referred to as a ‘population’. If the plasma concentration is lower than where the curve begins, 0% of the population will experience an effect. An effect of 50% means that the average effect in the total population is 50% of the maximum (and not a 50% effect in one individual) (Figure 10).
Unfortunately, most drugs have a Cp/response curve for side effects as well. This curve should be interpreted in the same way as Cp/response curves. The two curves together define the minimum and maximum plasma concentrations. The concentration that gives the minimum useful effect is the therapeutic threshold, while the plasma concentration at which the maximum tolerated side effects occur is called the therapeutic ceiling. Remember that Cp/response curves represent the dynamics in a group of patients, and can only offer a guideline when thinking in terms of an individual patient.