Tolerance to a drug is defined as the failure of a steady dose of the drug over time, to sustain the desired pharmacological effect, i.e., the need to increase the drug dosage to maintain the original pharmacological effect.
Tolerance can occur with a wide variety of drugs, including opioids.
Tolerance to the analgesic effects of opioids occur slowly, and some clinicians think it does not occur at all in cancer patients whose pain intensity remains stable. If pain seems to get rapidly out of control and large doses are required to bring the pain back under control, the pain intensity is probably increasing because of progression of the cancer. Addicted patients may be physically dependent on a drug, but physically dependent patients are not necessarily addicted to a drug. Physical dependence is a normal physiological response and should be anticipated in patients whose pain disappears and who must then be withdrawn from opioids. To avoid a withdrawal reaction, the opioid dose should be gradually reduced.