How soon is Mirena effective?

About.com states that Mirena is considered to be effective immediately after insertion, if the device is inserted within seven days of the start of the period. If it is placed at any other time in a woman's cycle, additional protection should be used for seven days following the insertion.

Mirena is a form of birth control known as an intrauterine device or IUD. About.com explains that it is a small device made of flexible plastic that resembles a T-shape. It is inserted into the uterus through the vagina. Insertion should be completed only by a recognized health care professional. Once in place, Mirena is active for up to five years.