This review summarizes evidence for sex differences in behavioral effects of opioids, primarily in rats. Whereas micro agonists have been found to be more potent and in some cases more efficacious in producing analgesia and sedation in males than females, females are more sensitive than males to reinforcing and locomotor stimulant effects of opioids. Sex differences in motoric effects of opioids may contribute to sex differences in other behavioral effects of opioids; for example, sex differences in rats' ability to discriminate morphine from saline can be attributed entirely to greater morphine-induced sedation in males. Chronic estradiol blunts females' sensitivity to morphine's analgesic and sedative effects, but enhances females' sensitivity to the reinforcing and locomotor stimulant effects of micro opioids. The neurobiological basis for sex differences in and estradiol modulation of behavioral effects of opioids includes brain opioid receptor density (greater in males and under low-estradiol conditions in females) and dopaminergic function (greater in females and under high-estradiol conditions). Given the significant and growing use of opioids by women, both medicinally and recreationally, understanding how female biology influences analgesic and other effects of opioids is crucial.
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