Nafarelin is a synthetic (man-made) protein that blocks the effects of the natural gonadotropin-releasing hormone (GnRH), a hormone that regulates the output of gonadotropins (a class of hormones) by the pituitary gland (a small gland located at the base of the brain). Gonadotropins (follicle stimulating hormone or FSH and luteinizing hormone or LH) are hormones that cause estrogen production by the ovaries. When nafarelin (known scientifically as a GnRH analog) is administered continuously to women in their reproductive years, the pituitary output of FSH, LH, and the production of estrogen by the ovaries are suppressed. Suppression of estrogen production causes menstruation to stop, resulting in a temporary (but reversible) state of "menopause." This temporary state of menopause results in shrinkage of the lining of the uterus, known as the endometrium.