Pharmacokinetic differences among opioids influence a patient's response to opioid treatment. An important element affecting a drug's pharmacokinetics, its metabolism, may be altered under various circumstances, thereby enhancing or mitigating a patient's response to opioids. The genetic background of the metabolic enzymes involved in opioid metabolism, comorbid medical conditions, older age, and the presence of other drugs that influence metabolism are such factors that can cause the response to opioid therapy to vary greatly from the expected response to a standard dose. As a result of the variability in individual responses to opioids, clinical management of pain with opioids must be empirical. © 2012 Published by Elsevier Inc. on behalf of U.S. Cancer Pain Relief Committee.
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