Millions of reproductive-age U.S. women experience premenstrual symptoms with varying degrees of severity. The large number and variety of premenstrual symptoms reported have made premenstrual disorders difficult to characterize. A number of mechanisms have been proposed to explain the etiology of premenstrual symptoms. Some women appear to have a genetic predisposition toward severe premenstrual symptoms or to have vulnerability traits that increase their risk. It has been suggested that 1 or more neurotransmitters and/or neurohormonal systems in certain women may have an abnormal response to normal fluctuations in gonadal hormones across the menstrual cycle. Premenstrual disorders can have a significant negative impact on a woman's quality of life and work productivity.