Norethisterone, also known as norethindrone and sold under many brand names, is a progestin medication used in birth control pills, menopausal hormone therapy, and for the treatment of gynecological disorders. The medication is available in both low-dose and high-dose formulations and both alone and in combination with an estrogen. It is used by mouth or, as norethisterone enanthate, by injection into muscle.Side effects of norethisterone include menstrual irregularities, headaches, nausea, breast tenderness, mood changes, acne, increased hair growth. Norethisterone is a progestin, or a synthetic progestogen, and hence is an agonist of the progesterone receptor, the biological target of progestogens like progesterone. It has weak androgenic and estrogenic activity, mostly at high dosages, and no other important hormonal activity.Norethisterone was discovered in 1951 and was one of the first progestins to be developed. It was first introduced for medical use on its own in 1957 and was introduced in combination with an estrogen for use as a birth control pill in 1963. It is sometimes referred to as a "first-generation" progestin. Along with desogestrel, it is one of the only progestins that is widely available as a progestogen-only "mini pill" for birth control. Norethisterone is marketed widely throughout the world. It is available as a generic medication. In 2019, it was the 143rd most commonly prescribed medication in the United States, with more than 4 million prescriptions. It is on the World Health Organizations List of Essential Medicines.
This might have been due to additional ethinylestradiol generated by higher doses of norethisterone. Overdose
There have been no reports of serious side effects with overdose of norethisterone, even in small children. As such, overdose usually does not require treatment. High dosages of as much as 60 mg/day norethisterone have been studied for extended treatment durations without serious adverse effects described. Interactions
5α-Reductase plays an important role in the metabolism of norethisterone, and 5α-reductase inhibitors such as finasteride and dutasteride can inhibit its metabolism. Norethisterone is partially metabolized via hydroxylation by CYP3A4, and inhibitors and inducers of CYP3A4 can significantly alter circulating levels of norethisterone. For instance, the CYP3A4 inducers rifampicin and bosentan have been found to decrease norethisterone exposure by 42% and 23%, respectively, and the CYP3A4 inducers carbamazepine and St. Johns wort have also been found to accelerate norethisterone clearance. Pharmacology
Pharmacodynamics
Norethisterone is a potent progestogen and a weak androgen and estrogen. That is, it is a potent agonist of the progesterone receptor (PR) and a weak agonist of the androgen receptor (AR) and the estrogen receptor (ER). Norethisterone itself has insignificant affinity for the ER; its estrogenic activity is from an active metabolite that is formed in very small amounts, ethinylestradiol (EE), which is a very potent estrogen. Norethisterone and its metabolites have negligible affinity for the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) and hence have no glucocorticoid, antiglucocorticoid, mineralocorticoid, or antimineralocorticoid activity. Progestogenic activity
Norethisterone is a potent progestogen and binds to the PR with approximately 150% of the affinity of progesterone.