A sex difference in the interaction between promethazine and morphine in the mouse.

The effects of promethazine on the antinociceptive and respiratory actions of morphine have been examined in the mouse. Moderate doses of promethazine (5 and 10 mg kg-1) potentiated morphine's action in male mice but inhibited it in female mice. Gonadectomy abolished the interaction between promethazine and morphine in both sexes, although the intensity and duration of morphine's activity was greatly enhanced in these mice. Replacement of oestradiol in ovariectomized mice restored morphine's activity to intact female control values. However, interactions between promethazine and morphine required progesterone, as well as oestradiol, replacement to obtain results approaching those obtained in intact female mice.