Figure 1. Hypothetical models of drug tolerance.

In drug tolerance, increased amounts of drug are needed to maintain an effect. In the clinical setting, many patients with malignant pain stabilize their dose over prolonged periods of time, with the need for further increases due to the progression of their underlying disease process. Tolerance is often considered as a progressive effect, in which drug doses need to be continually escalated. However, clinical observations suggest that in many situations tolerance can achieve a steady-state in which stable doses of drug are sufficient for extended periods of time.