[8] As an emergency birth control, sold under the brand names Plan B One-Step and Julie, among others, it is useful within 72 hours of unprotected sex.

[12] Common side effects include nausea, breast tenderness, headaches, and increased, decreased, or irregular menstrual bleeding.

[21] Levonorgestrel is used in emergency contraceptive pills (ECPs), both in a combined Yuzpe regimen which includes estrogen, and as a levonorgestrel-only method.

[medical citation needed] However, taking more than one dose of emergency contraception does not increase the chance of pregnancy not happening.

"[22] The primary mechanism of action of levonorgestrel as a progestogen-only emergency contraceptive pill is, according to International Federation of Gynecology and Obstetrics (FIGO), to prevent fertilization by inhibition of ovulation and thickening of cervical mucus.

[23][24][25][26] FIGO has stated that: "review of the evidence suggests that LNG [levonorgestreol] ECPs cannot prevent implantation of a fertilized egg.

"[27][28] In November 2013, the European Medicines Agency (EMA) approved a change to the label saying it cannot prevent implantation of a fertilized egg.

"[29][32][33] In November 2013 and January 2014, the FDA and the EMA said they were reviewing whether increased weight and body mass index (BMI) reduce the efficacy of emergency contraceptives.

[38] Estradiol with levonorgestrel in the form of a skin patch is used under the brand name Climara Pro for hormone replacement therapy in postmenstrual women, treating symptoms such as hot flashes or osteoporosis.

[40][41] The levonorgestrel intrauterine system (LNG-IUS) is a type of long-term birth control that releases the progestin into the uterine cavity.

[42][21] Levonorgestrel is released at a constant, gradual rate of 0.02 mg per day by the polydimethylsiloxane membrane of the device, which renders it effective for up to five years.

[42] This high level of levonorgestrel impedes the function of the endometrium, making it hostile for sperm transport, fertilization, and implantation of an ovum.

[44] After an intake of 1.5 mg levonorgestrel in clinical trials, very common side effects (reported by 10% or more) included: hives, dizziness, hair loss, headache, nausea, abdominal pain, uterine pain, delayed menstruation, heavy menstruation, uterine bleeding, and fatigue; common side effects (reported by 1% to 10%) included diarrhea, vomiting, and painful menstruation; these side effects usually disappeared within 48 hours.

[45][46] However, the long term side effects common with oral contraceptives such as arterial disease are lower with levonorgestrel than in combination pills.

[medical citation needed] Levonorgestrel as a contraceptive intrauterine device is associated with a higher risk of breast cancer than with non-use.

[48] These include, but are not limited to barbiturates, bosentan, carbamazepine, felbamate, griseofulvin, oxcarbazepine, phenytoin, rifampin, St. John's wort and topiramate.

[57][58][59] Levonorgestrel stimulates the proliferation of MCF-7 breast cancer cells in vitro, an action that is independent of the classical PRs and is instead mediated via the progesterone receptor membrane component-1 (PGRMC1).

[71][72] Norgestrel (rac-13-ethyl-17α-ethynyl-19-nortestosterone), the racemic mixture containing levonorgestrel and dextronorgestrel, was discovered by Hughes and colleagues at Wyeth in 1963 via structural modification of norethisterone (17α-ethynyl-19-nortestosterone).

[14][75][76] Levonorgestrel was first studied in humans by 1970, and was introduced for medical use in Germany as a combined birth control pill with ethinylestradiol under the brand name Neogynon in August 1970.

[19][83][84] In addition to combined formulations, levonorgestrel was introduced as a progestogen-only pill under the brand names Microlut by 1972 and Microval by 1974.

[19][95] In 2013, the Food and Drug Administration approved Plan B One-Step for sale over-the-counter in the United States without a prescription or age restriction.

[19][63][64][85] Levonorgestrel is marketed alone or in combination with an estrogen (specifically ethinylestradiol, estradiol, or estradiol valerate) under a multitude of brand names throughout the world, including Alesse, Altavera, Alysena, Amethia, Amethyst, Ashlyna, Aviane, Camrese, Chateal, Climara Pro, Cycle 21, Daysee, Emerres, Enpresse, Erlibelle, Escapelle, Falmina, Introvale, Isteranda, Jadelle, Jaydess, Jolessa, Klimonorm, Kurvelo, Kyleena, Lessina, Levlen, Levodonna, Levonelle, Levonest, Levosert, Levora, Liletta, Loette, Logynon, LoSeasonique, Lutera, Lybrel, Marlissa, Microgynon, Microlut, Microvlar, Min-Ovral, Miranova, Mirena, My Way, Myzilra, Next Choice, Nordette, Norgeston, NorLevo, Norplant, One Pill, Option 2, Orsythia, Ovima, Ovranette, Plan B, Plan B One-Step, Portia, Postinor, Postinor-2, Preventeza, Ramonna, Rigevidon, Quartette, Quasense, Seasonale, Seasonique, Skyla, Sronyx, Tri-Levlen, Trinordiol, Triphasil, Triquilar, Tri-Regol, Trivora, and Upostelle, among many others.

[100] A policy update in 2015, required all pharmacies, clinics, and emergency departments run by Indian Health Services (for Native Americans) to have Plan B One-Step in stock, to distribute it to any woman (or her representative) who asked for it without a prescription, age verification, registration or any other requirement, to provide orientation training to all staff regarding the medication, to provide unbiased and medically accurate information about emergency contraception, and to make someone available at all times to distribute the pill in case the primary staffer objected to providing it on religious or moral grounds.

[52][54] Mechanism of actionCopper-releasing IUCsWhen used as a regular or emergency method of contraception, copper-releasing IUCs act primarily to prevent fertilization.

Emergency insertion of a copper IUC is significantly more effective than the use of ECPs, reducing the risk of pregnancy following unprotected intercourse by more than 99%.2,3 This very high level of effectiveness implies that emergency insertion of a copper IUC must prevent some pregnancies after fertilization.Emergency contraceptive pillsTo make an informed choice, women must know that ECPs—like the birth control pill, patch, ring, shot, and implant,76 and even like breastfeeding77—prevent pregnancy primarily by delaying or inhibiting ovulation and inhibiting fertilization, but may at times inhibit implantation of a fertilized egg in the endometrium.

Pregnancy begins with implantation according to medical authorities such as the US FDA, the National Institutes of Health79 and the American College of Obstetricians and Gynecologists (ACOG).80Ulipristal acetate (UPA).

Several clinical studies have shown that combined ECPs containing ethinyl estradiol and levonorgestrel can inhibit or delay ovulation.107–110How does EC work?In 2002, a judicial review ruled that pregnancy begins at implantation, not fertilisation.8 The possible mechanisms of action should be explained to the patient as some methods may not be acceptable, depending on individual beliefs about the onset of pregnancy and abortion.Copper-bearing intrauterine device (Cu-IUD).

Copper is toxic to the ovum and sperm and thus the copper-bearing intrauterine device (Cu-IUD) is effective immediately after insertion and works primarily by inhibiting fertilisation.9–11 A systematic review on mechanisms of action of IUDs showed that both pre- and postfertilisation effects contribute to efficacy.11 If fertilisation has already occurred, it is accepted that there is an anti-implantation effect,12,13Levonorgestrel (LNG).

The precise mode of action of levonorgestrel (LNG) is incompletely understood but it is thought to work primarily by inhibition of ovulation.16,17Ulipristal acetate (UPA).

The primary mechanism of action is to stop or disrupt ovulation; LNG ECP use may also prevent the sperm and egg from meeting.16Emergency postcoital contraceptionLevonorgestrelMechanism and efficacyLevonorgestrel-only emergency contraceptive pills:• Interfere with the process of ovulation;• May possibly prevent the sperm and the egg from meeting.Implications of the research:• Inhibition or delay of ovulation is LNG ECPs principal and possibly only mechanism of action.• Review of the evidence suggests that LNG-ECs cannot prevent implantation of a fertilized egg.