Etonogestrel is a medication which is used as a means of birth control for women. It is available as an implant placed under the skin of the upper arm under the brand names Nexplanon and Implanon, and in combination with ethinylestradiol, an estrogen, as a vaginal ring under the brand names NuvaRing and Circlet. Etonogestrel is effective as a means of birth control and lasts at least three or four years with some data showing effectiveness for five years. Following removal, fertility quickly returns. Side effects of etonogestrel include menstrual irregularities, breast tenderness, mood changes, acne, headaches, vaginitis, and others. Etonogestrel is a progestin, or a synthetic progestogen, and hence is an agonist of the progesterone receptor, the biological target of progestogens like progesterone. It works by stopping ovulation, thickening the mucus around the opening of the cervix, and altering the lining of the uterus. It has very weak androgenic and glucocorticoid activity and no other important hormonal activity. Etonogestrel was patented in 1972 and introduced for medical use in 1998. It became available in the United States in 2006. Etonogestrel implants are approved in more than 90 countries and used by about three million women globally as of 2010. A closely related and more widely known and used progestin, desogestrel, is a prodrug of etonogestrel in the body. Medical uses Etonogestrel is used in hormonal contraception in form of the etonogestrel contraceptive implant and the contraceptive vaginal ring (brand names NuvaRing, Circlet), the latter in combination with ethinylestradiol. Etonogestrel birth control implants are a type of long-acting reversible contraception, which has been shown to be one of the most effective form of birth control. The failure rate of the implants is 0.05% for both perfect use and typical use because the method requires no user action after placement. Studies of one type, which include over 2,467 women-years of exposure, found no pregnancies. Other studies have found some failures with this method, some attributed to failures of the method itself and others to improper placement, drug interactions, or conception prior to method insertion. In comparison, tubal sterilization has a failure rate of 0.5% and IUDs have a failure rate of 0.2–0.8%. A single implant is approved for three years with data showing effectiveness for five years. Contraindications Women should not use implants if they: are, or think they are, pregnant are allergic to etonogestrel have vaginal bleeding that has not