Mirena is a hormone-releasing system placed in your uterus to prevent pregnancy for as long as you want for up to 5 years. Mirena also treats heavy periods in women who choose intrauterine contraception. Mirena is a safe and effective birth control, made of soft, flexible plastic that's put in place by your healthcare provider during an office visit. It is over 99% effective at preventing pregnancy and is one of the most effective forms of birth control. It can be removed by your healthcare provider at any time during the 5-year period. With Mirena, there’s no need to remember to use birth control. Simply check the threads once a month - your healthcare provider will show you how.