A long acting, hassle free, and highly effective method of birth control. Releases a small amount of hormone (levonorgestrel) that causes the cervical mucus to thicken which prevents sperm from entering the cervix. May also prevent ovulation and fertilization. Can result in decreased period flow and may protect against uterine fibroids and cancer of the uterus, cervix and ovaries. Has a .1% to .4% (1 to 4 in 1,000) failure rate. Can be left in for up to 3 years.