Sex differences in pain perception and in response to opioids have been described, but the findings are inconsistent. We sought to determine the effect of sex on pain perception, morphine consumption, and morphine analgesia after surgery. We designed a prospective cohort study and included 423 women and 277 men who emerged from general anesthesia after surgical procedures and who reported pain intensity of >or=5 on the 0-10 numeric rating scale (NRS). We administered 2.5 mg of morphine IV every 10 min until the pain intensity was <or=4 of 10. Every 10 min, patients rated their pain on the NRS and indicated the degree of pain relief on a 5-point Likert scale. After adjustment for type of operation and age, we found that women had more intense pain and had larger morphine consumption than men. The difference in NRS pain intensity was 0.4 U (95% confidence interval, 0.1-0.6 U). Women required 0.03 mg/kg more morphine than men (95% confidence interval, 0.02-0.04 mg/kg). We conclude that women have more intense pain and require 30% more morphine to achieve a similar degree of analgesia compared with men. Clinicians should anticipate the differences in opioid requirement to avoid undertreatment of pain in women.
Implications: The effect of sex on opioid response is not clear. To determine the effect of sex on morphine consumption and morphine analgesia, we designed a cohort study. We found that women had more intense pain and required 30% more morphine to achieve a similar degree of analgesia compared with men.