A hormone that works by causing changes in the uterus. When taken with the hormone estrogen, progestin works to prevent thickening of the lining of the uterus. This is helpful for women who are in menopause and are taking estrogen for their symptoms. Progestins also are prescribed to regulate the menstrual cycle, treat unusual stopping of the menstrual periods, help a pregnancy occur or maintain a pregnancy, or treat unusual or heavy bleeding of the uterus. They also can be used to prevent pregnancy, help treat cancer of the breast, kidney, or uterus, and help treat loss of appetite and severe weight or muscle loss.