Opioids are the mainstay of treatment for moderate to severe pain. However, opioid therapy in the elderly is often associated with significant morbidity because of excessive ventilatory depression. The large amount of interindividual variability in opioid dose-response relationships makes it difficult to individualize the dose and dosing interval to provide safe and effective analgesia. By examining how aging affects the pharmacokinetics (PK) and pharmacodynamics (PD) of opioids, it is possible to provide a rational basis for age adjustment in opioid dosing.