Chemistry
Levonorgestrel (levo=left) is one form of a hormone that exists in two mirror image left and right forms.
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.
If taken together with drugs that induce the CYP3A4 cytochrome liver enzyme, the levonorgestrel will be metabolized faster and have lower efficacy, and a larger dose will be needed.
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