Hi, Mirena is a continuous slow release type of IUD, which releases a hormone named Levonorgestrel, a type of progestin, which acts as a non-contraceptive device for a longer period, may be up to 8 years. Though, its side effect on breast is not commonly seen, as the amount release of progestin at a time is small, still once a while, due to the action of progestin on breasts, it may give rise to some tenderness or soreness of breasts. However, this effect is usually transient due to changed levels of hormones in the body and it may go by itself within a short period, when the body adjusts to the changed hormone levels. Still, if the problem persists for a long time or it becomes aggravated, then you are advised to consult your doctor and treatment done accordingly. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Satnarain Bansal, General & Family Physician