Norethisterone is a progestin, meaning it is a synthetic version of a type of hormone called progestogen that is produced naturally in the human body by the ovaries and placenta. The most common natural form of progestogen is progesterone, which affects several functions related to the female reproductive system, like pregnancy, embryo development, and menstruation. In the body, norethisterone works much like natural progesterone, and is used in oral birth control, to treat painful and irregular menstruation, to delay a period, and to treat women who experience very heavy bleeding during their period. It is also used to treat premenstrual syndrome (PMS) and postmenopausal syndrome. At higher doses, it can be used as a part of breast cancer treatment.
This synthetic hormone was successfully manufactured as early as 1951, and was used in one of the first types of birth control pill to come on the market in the 1960s. As a birth control medication, norethisterone can prevent pregnancy by preventing ovulation, meaning the release of an egg from the ovaries, but it is not guaranteed to do so in all women. It also thickens the mucus in the cervix, the narrow part of the uterus where it joins the vagina, and affects the lining of the uterus itself. The thickened mucus in the cervix makes it harder for sperm to enter the uterus to fertilize an egg, and the effect on the lining of the uterus makes it more difficult for a fertilized egg to attach itself and develop.
Norethisterone is a common active ingredient in a type of oral contraceptive called the progestogen-only pill (POP), or the mini-pill. It can also be used in combined oral contraceptive pills that include progestogen and another type of hormone called estrogen. Combined oral contraceptive pills prevent ovulation more effectively than norethisterone by itself, but there is an increased risk of blood clots forming in the veins using this type of contraception. Such blood clots are rare, but can be fatal if they are transported by the circulatory system to the lungs. One should consult a physician immediately if experiencing severe headaches, sudden vision changes, swelling of the legs, or chest pains while taking norethisterone.
Common side effects of norethisterone include tender breasts, bloating, and bleeding between regular periods. Less common side effects include nausea, headaches, weight gain, and dizziness. Those with a history of stroke or heart attack or those with liver problems or kidney problems should not take norethisterone.