Morphine responses in humans: a retrospective analysis of sex differences.

There is increasing evidence that sex modulates the effects of opioid analgesics in nonhumans, but few studies have examined this issue in humans. Over the past seven years we have conducted several studies in which the subjective, psychomotor, and physiological effects of intravenous morphine were examined in healthy volunteers. In a retrospective analysis encompassing six studies, we re-examined the effects of 10 mg/70 kg (iv) morphine in 57 males and 27 females. There were some differences in morphine's subjective effects as a function of sex. Females reported higher ratings of 'coasting (spaced out),' 'heavy or sluggish feeling' and 'dry mouth.' No differences in degree of psychomotor impairment or physiological effects (miosis and respiration rate) of morphine emerged between males and females. Future studies should focus on other doses of morphine and other opioid drugs, assess multiple behavioral and physiological endpoints, and look at different subsamples of humans (e.g. opioid abusers).