[2] It is used for birth control, heavy menstrual periods, and to prevent excessive build of the lining of the uterus in those on estrogen replacement therapy.

[2] Side effects include irregular periods, benign ovarian cysts, pelvic pain, and depression.

[5] It works by thickening the mucus at the opening of the cervix, stopping the buildup of the lining of the uterus, and occasionally preventing ovulation.

[7][8] The hormonal IUD is an extremely effective method of birth control, and a 2021 study demonstrated that it may be used for emergency contraception.

The hormonal IUD is considered to be more effective than other common forms of reversible contraception, such as the birth control pill, because it requires little action by the user after insertion.

[23] In a 10-year study, the levonorgestrel IUD was found to be as effective as oral medicines (tranexamic acid, mefenamic acid, combined oestrogen–progestogen or progesterone alone) for heavy periods; the same proportion of study participants had not had surgery for heavy bleeding and had similar improvements in their quality of life.

[42] The World Health Organization recommends against immediate postpartum insertion, citing increased expulsion rates.

The IARC in 1999 concluded that there is no evidence progestin-only birth control increases the risk of any cancer, though the available studies were too small to be definitively conclusive.

[68] Researchers cautioned against causal interpretation from this study, citing confounding effects, methodological concerns and a 2020 meta-analysis of randomized controlled trials which showed no increased risk.

[73][74] One of the studies showed at seven years of use, similar BMD at the midshaft of the ulna and at the distal radius as nonusers matched by age and BMI.

The authors of the study said their results were predictable, since it is well established that the main factor responsible for bone loss in women is hypoestrogenism, and, in agreement with previous reports, they found estradiol levels in Mirena users to be normal.

The string allows physicians and patients to check to ensure the IUD is still in place and enables easy removal of the device.

[88] Other effects include making the inside of the uterus become fatal to sperm[86][89] and thinning of the endometrial lining, but this is not the usual function.

[94] Dr. Antonio Scommenga, working at the Michael Reese Hospital in Chicago, discovered that administering progesterone inside the uterus could have contraceptive benefits.

[94] With knowledge of Scommegna's work, a Finnish doctor, Jouni Valter Tapani Luukkainen, created the T-shaped IUD that released progesterone, marketed as the Progestasert System in 1976.

Following this relative lack of success, Dr. Luukkainen replaced the progesterone with the hormone levonorgestrel to be released over a five-year period, creating what is now Mirena.

[97] In 2009, Bayer, the maker of Mirena, was issued an FDA Warning Letter by the United States Food and Drug Administration for overstating the efficacy, minimizing the risks of use, and making "false or misleading presentations" about the device.

Fact: IUCs prevent fertilization and are true contraceptives.ConclusionsActive substances released from the IUD or IUS, together with products derived from the inflammatory reaction present in the luminal fluids of the genital tract, are toxic for spermatozoa and oocytes, preventing the encounter of healthy gametes and the formation of viable embryos.

The common belief that the usual mechanism of action of IUDs in women is destruction of embryos in the uterus is not supported by empirical evidence.

The devices also create barriers to sperm transport and fertilization, and sensitive assays detect hCG in less than 1% of cycles, indicating that significant prevention must occur before the stage of implantation.Mechanism of actionThe contraceptive action of all IUDs is mainly in the intrauterine cavity.

It is believed that this reaction, a sterile inflammatory response, produces tissue injury of a minor degree but sufficient to be spermicidal.

Very few, if any, sperm reach the ovum in the fallopian tube.The progestin-releasing IUD adds the endometrial action of the progestin to the foreign body reaction.

The endometrium becomes decidualized with atrophy of the glands.65 The progestin IUD probably has two mechanisms of action: inhibition of implantation and inhibition of sperm capacite journal, penetration, and survival.Intrauterine devicesMechanisms of actionThe common belief that the usual mechanism of action of IUDs in women is destruction of embryos in the uterus is not supported by empirical evidence... Because concern over mechanism of action represents a barrier to acceptance of this important and highly effective method for some women and some clinicians, it is important to point out that there is no evidence to suggest that the mechanism of action of IUDs is abortifacient.The LNG-IUS, like the copper device, has a very low ectopic pregnancy rate.

Less inflammation occurs within the uterus of LNG-IUS users, but the potent progestin effect thickens cervical mucus to impede sperm penetration and access to the upper genital track.