Norethisterone

BRAND NAMES

Norethisterone is marketed under several brand names worldwide, including:

• Primolut N (one of the most common brands)

• Noriday

• Micronor

• Aygestin

• Norlutin

MECHANISM OF ACTION

Norethisterone works by binding to progesterone receptors in the reproductive system, mimicking the action of natural progesterone. It suppresses ovulation by inhibiting the release of gonadotropins (LH and FSH), thickens cervical mucus to prevent sperm penetration, and stabilizes the endometrial lining, which helps regulate menstrual cycles and reduce abnormal uterine bleeding.

PHARMACOKINETICS

Absorption

Norethisterone is well absorbed after oral administration, but it undergoes significant first-pass metabolism in the liver, which reduces its systemic bioavailability. Peak plasma concentrations are usually reached within a few hours, and food has minimal effect on overall absorption, so it can be taken with or without meals.

Distribution

Norethisterone is widely distributed in the body after absorption due to its lipophilic nature. It is highly bound to plasma proteins, mainly albumin and sex hormone–binding globulin (SHBG), which limits the free active fraction in circulation. It also distributes into reproductive tissues, where it exerts its primary hormonal effects.

Metabolism

Norethisterone is extensively metabolized in the liver, mainly through reduction and conjugation pathways, including conversion to inactive metabolites and partial transformation to ethinylestradiol in small amounts. Hepatic enzymes, especially those involved in steroid metabolism, play a key role in its breakdown, and the metabolites are then prepared for excretion.

Elimination

Norethisterone is eliminated mainly through the kidneys as inactive metabolites, with a smaller amount excreted in feces via bile. The parent drug is present in very low amounts in urine due to extensive hepatic metabolism. It has a moderate elimination half-life, allowing for once- or twice-daily dosing depending on the indication.

PHARMACODYNAMICS

Norethisterone produces its pharmacodynamic effects by binding to progesterone receptors and mimicking natural progesterone activity. It suppresses the hypothalamic–pituitary–ovarian axis by inhibiting LH and FSH release, thereby preventing ovulation. It also causes thickening of cervical mucus, making sperm penetration difficult, and stabilizes the endometrial lining, which reduces abnormal uterine bleeding and regulates menstrual cycles.

ADMINISTRATION

Norethisterone is administered orally in tablet form. It is usually taken once or multiple times daily depending on the indication, such as menstrual disorders, endometriosis, or contraception. The tablets should be taken at the same time each day for consistent hormonal effect, and dosing schedules vary based on the clinical condition and physician prescription.

DOSAGE AND STRENGTH

Norethisterone is available in oral tablet strengths of 5 mg, 10 mg, and 0.35 mg (mini-pill formulation in some contraceptives). The dosage varies depending on the indication: for menstrual disorders it is commonly 5–10 mg taken 2–3 times daily, while for contraception it is typically 0.35 mg once daily in continuous dosing. The exact regimen is adjusted based on the condition being treated and patient response.

DRUG INTERACTIONS

Norethisterone has several important drug interactions:

• Enzyme inducers (CYP3A4) (e.g., rifampin, carbamazepine, phenytoin, St. John’s wort) → decrease effectiveness by increasing metabolism

• Antiepileptics → may reduce hormone levels and contraceptive efficacy

• Antiretrovirals → variable effects, may reduce or increase hormone levels depending on agent

• Anticoagulants (e.g., warfarin) → may alter anticoagulant effect, requiring monitoring

• Glucocorticoids→ may have altered metabolism when combined

FOOD INTERACTIONS

Norethisterone has minimal food interactions and can be taken with or without food. Food does not significantly affect its absorption or effectiveness, though taking it with meals may help reduce mild stomach discomfort in some patients.

CONTRAINDICATIONS

Norethisterone is contraindicated in patients with hypersensitivity to the drug, active or history of thromboembolic disorders (such as deep vein thrombosis or pulmonary embolism), severe liver disease or liver tumors, and undiagnosed vaginal bleeding. It should also be avoided in known or suspected breast or hormone-dependent cancers. Pregnancy is a contraindication for its use in non-therapeutic situations.

SIDE EFFECTS

• Mood changes 

• Irregular menstrual bleeding 

• Bloating 

• Acne 

• Dizziness 

• Fluid retention 

• Decreased libido

OVER DOSAGE

• Nausea and vomiting 

• Dizziness 

• Breast tenderness 

• Headache 

• Vaginal bleeding or spotting 

• Mood changes 

• Fluid retention

TOXICITY

Norethisterone toxicity is rare and usually causes hormonal side effects like nausea, headache, dizziness, breast tenderness, mood changes, and irregular bleeding. In rare cases, it may increase the risk of blood lots or liver issues. Treatment is supportive with stopping the drug and managing symptoms.