Intrauterine device (IUD) levonorgestrel

A T-shaped plastic device inserted into the uterus that steadily releases small amounts of levonorgestrel each day

How it works: 
Suppresses the growth of the lining of uterus (endometrium)
Effectiveness to prevent pregnancy: 
>99%
Comments: 
Decreases amount of blood lost with menstruation over time; Reduces menstrual cramps and symptoms of endometriosis; amenorrhea (no menstrual bleeding) in a group of users