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