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The dose of opioid prescribed for postoperative pain relief has traditionally been based on the weight of the patient. Although a reduction in dose is often suggested for elderly patients over 70 years of age, age-related alterations to dose are generally not considered for younger patients. The records of 1010 patients, under 70 years old, prescribed morphine via patient-controlled analgesia (PCA) after major operations were examined to see what factors might best predict the amount of morphine used in the first 24 h after surgery. Factors included were age, sex, weight, operative site, verbal numeric pain score (at rest and on movement) and a nausea/vomiting score. In a subgroup of 78 of these patients, the effects of intraoperative and recovery room doses of opioid (‘clinical’ loading dose) were analysed.Although the interpatient variability in PCA morphine doses was large (differences of up to 10-fold in each age group), the best predictor of PCA morphine requirement in the first 24 h after surgery (the amount required in the 24 h after the initial loading dose) was the age of the patient. An estimate of these requirements for patients over the age of 20 years can be obtained from the formula: average first 24 h morphine requirement (mg) = 100 − age.PCA allows patients the flexibility to titrate their own opioid dose; if conventional analgesic regimens are to become more effective, they too need to allow for the wide interpatient variation in dose requirements. Although previous studies have noted a correlation between patient age and the amount of opioid needed, this study quantifies this correlation and provides guidelines for opioid dosing. Prescriptions for conventional analgesic regimens should include a dose range centred on values obtained from the above formula to allow for the large interpatient variation in each age group. While initial morphine dose should be guided by patient age and not weight, subsequent doses must still be titrated according to effect.