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Levonorgestrel is a manufactured hormone used in a number of birth control methods. In pill form, sold under the brand name Plan B among others, it is useful within 120 hours as emergency birth control. It becomes less effective the longer after sex and only works before pregnancy has occurred. It is also combined with an estrogen to make combined oral birth control pill. Within an IUD, sold as Mirena among others, it is effective for long term prevention of pregnancy. An implantable form of levonorgestrel is also available in some countries.
Common side effects include nausea, breast tenderness, headaches, and increased, decreased, or irregular menstrual bleeding. If used as a form of emergency contraception during pregnancy there is no evidence it affects the baby. It is safe to use during breastfeeding. Birth control that contains levonorgestrel will not change the risk of sexually transmitted infections. It works mostly by decreasing ovulation and closing off the cervix to prevent the passage of sperm.
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