Predictive factors and opioid responsiveness in cancer pain.

The management of cancer pain not readily responsive to morphine is often problematic. Several factors can interfere with an appropriate analgesic opioid response in the course of the illness, including the progression of the disease and tolerance, the appearance of intractable side-effects, type and temporal pattern of pain, morphine metabolites, pharmacokinetic and pharmacodynamic factors, as well as individual factors. Different methodologies capable of accurately predicting or monitoring opioid response have been proposed in an attempt to allow researchers to 'speak a common language'. Tolerance is a component of the concept of opioid responsiveness. However, the assessment of analgesic tolerance in cancer patients is constrained by numerous difficulties because of the changes in the noxious stimuli with increasing activity in nociceptive pathways.