It is clear that there is a significant clinical need for uterine relaxants in the treatment of dysmenorrhea and preterm labor. However, the etiologies of these disorders are usually unknown and likely to be multifactorial, so there is no rational basis for therapy with a single group of drugs where intervention is indicated. Current ideas about the mechanism of dysmenorrhea ascribe a major role to increased secretion of vasopressin and various prostanoids. 1 , 2 Theories of parturition can be divided into those assigning a role to

increased secretion of spasmogens (agents such as oxytocin and prostanoids that interact with myometrial cells and cause contractions [or spasms] or increase ongoing contractions)

increased sensitivity of myometrium to preexisting spasmogens (e.g., greater oxytocin receptor numbers)

reduced secretion of a preexisting inhibitory influence (e.g., progesterone block theory).