Molecular Formula: C21H28O2 
Formula Weight: 312.45
CAS: 797-63-7
18,19-Dinorpregn-4-en-20-yn-3-one,13-ethyl-17-hydroxy-,(17a)-(-)-.
(-)-13-Ethyl-17-hydroxy-18,19-dinor-17a-pregn-4-en-20-yn-3-one

Levonorgestrel (or l-norgestrel or D-norgestrel) is a second generation synthetic progestogen used as an active ingredient in some hormonal contraceptives

Chemistry
Chemically, it is a hormonally active levorotatory enantiomer of the racemic mixture norgestrel. It is a gonane progestin derived from 19-nortestosterone.

Its in vitro relative binding affinities at human steroid hormone receptors are: 323% that of progesterone at the progesterone receptor, 58% that of testosterone at the androgen receptor, 17% that of aldosterone at the mineralocorticoid receptor, 7.5% that of cortisol at the glucocorticoid receptor, and <0.02% that of estradiol at the estrogen receptor.

Usage
Oral contraceptionAt low doses, levonorgestrel is used in monophasic and triphasic formulations of combined oral contraceptive pills, with available monophasic doses ranging from 100-250 µg, and triphasic doses of 50 µg/75 µg/125 µg.

At very low daily dose of 30 µg, levonorgestrel is used in some progestogen only pill formulations.

Emergency contraception
Levonorgestrel is used in emergency contraceptive pills (ECPs), both in a combined Yuzpe regimen which includes estrogen, and as a levonorgestrel-only method. The levonorgestrel-only method uses levonorgestrel 1500 μg (as a single dose or as two 750 μg doses 12 hours apart) taken within 3 days of unprotected sex, with one study indicating that beginning as late as 120 hours (5 days) after intercourse could be effective. There are many brand names of levonorgestrel-only ECPs, including: Escapelle, Plan B, Levonelle, NorLevo, Postinor-2, i-pill, "Next Choice" and 72-HOURS.

Intrauterine system
Levonorgestrel is the active ingredient in the Mirena intrauterine system.

Contraceptive implants
Levonorgestrel is the active ingredient in Norplant and Jadelle.

Side effects
Possible side effects of levonorgestrel include nausea, vomiting, stomach pain, dizziness, breast tenderness, tiredness and weakness, headache, menstrual changes, and diarrhea.

It decreases total and free testosterone, androstenedione, dehydroepiandrosterone sulfate (DHEAS), dihydrotestosterone (DHT) and sex hormone–binding globulin (SHBG), but has no effect on sexual function or markers of androgen bioactivity.

What is levonorgestrel intrauterine system?
Levonorgestrel is a female hormone. This hormone can cause changes in your cervical mucus and uterine lining, making it harder for sperm to reach the uterus and harder for a fertilized egg to attach to the uterus.

Levonorgestrel intrauterine system is a plastic device that contains the hormone levonorgestrel. This device is placed in the uterus where it slowly releases the hormone to prevent pregnancy for up to 5 years. Levonorgestrel intrauterine system is meant for use in a woman who has had at least one child and is in a stable sexual relationship with someone who has no other sexual partners.

Levonorgestrel intrauterine system is also used in women who have heavy menstrual bleeding and choose to use an intrauterine form of birth control.

Levonorgestrel is a progestin hormone and does not contain estrogen. The intrauterine device releases levonorgestrel into the uterus, but only small amounts of the hormone reach your blood stream.
Levonorgestrel intrauterine system may also be used for other purposes not listed in this medication guide.