Progestin is a synthetic hormone used to affect a woman's body in the same ways as the hormone progesterone. The ovaries produce progesterone during the second half of the menstrual cycle to help prepare the uterus for pregnancy; at the time of menopause, the level of progesterone declines.

Progesterone regulates the cyclic growth and breakdown of the uterine lining (endometrium). When a woman's progesterone levels are low, progestin can be used to treat endometrial problems such as heavy, irregular menstrual bleeding. Women with an intact uterus who take estrogen are also prescribed progestin, which prevents the estrogen from causing cancerous cell growth in the endometrium.