Paradoxical effect of flavoxate on human uterine contractility in vivo

Flavoxate's effect on uterine contractility was investigated in 20 women with histories of primary dysmenorrhea. After 100 mg of the smooth muscle relaxant was infected intravenously, intrauterine pressure was recorded continuously for 1--2 hours. Immediately after the injection, intrauterine pressure and the frequency of contractions increased for 5--15 minutes at all phases of the menstrual cycle. After the activating response, there was a reduction in both the frequency of contractions and uterine tonus below preinjection levels in some patients. The paradoxical effect of flavoxate on uterine motility may be caused by its relaxation of vascular smooth muscle, which results in increased blood flow and, consequently, increased oxygen supply to the myometrium.