Norethisterone

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Norethisterone on Prescriptsy

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Norethisterone is a synthetic progestogen (progestin) that has been used in clinical practice for over 50 years.

It is available as 5 mg tablets and is prescribed for a range of gynaecological conditions, including period delay, heavy menstrual bleeding (menorrhagia), dysfunctional uterine bleeding, endometriosis, and premenstrual syndrome.

Norethisterone works by mimicking the effects of the natural hormone progesterone on the uterine lining and the hormonal feedback systems that regulate the menstrual cycle.

It is one of the most commonly prescribed progestogens in the United Kingdom and is a prescription-only medicine (POM) available on the NHS.

Menstrual disorders are among the most frequent reasons women consult their GP.

Heavy menstrual bleeding alone affects approximately 1 in 4 women of reproductive age and is associated with iron-deficiency anaemia, fatigue, reduced quality of life, and time off work or school.

Period delay is one of the most popular and straightforward uses of norethisterone, commonly requested before holidays, weddings, examinations, and other events.

This page provides a comprehensive clinical overview of norethisterone 5 mg, including how it works, dosage guidance for each indication, side effects, important safety warnings, and how to obtain a prescription in the United Kingdom.

Important safety information about norethisterone

• Norethisterone 5 mg tablets are a prescription-only medicine and are not a contraceptive.
• At high doses (10 mg or more daily), norethisterone is partly converted to ethinylestradiol in the body, which may increase the risk of blood clots (VTE).
• Do not take norethisterone if you are pregnant or suspect you may be pregnant.
• If you develop sudden severe headache, leg pain or swelling, chest pain, or visual disturbance while taking norethisterone, stop the tablets and seek urgent medical help.
• Norethisterone is not the same as the progestogen-only pill (Noriday), which contains a much lower dose (350 micrograms) and is used for contraception.

Understanding progestogens and the menstrual cycle

The menstrual cycle is regulated by a complex interplay of hormones, principally oestrogen and progesterone.

During the first half of the cycle (the follicular phase), rising oestrogen levels stimulate the growth and thickening of the endometrium (womb lining).

After ovulation, the corpus luteum produces progesterone, which transforms the endometrium from a proliferative to a secretory state, preparing it for potential implantation of a fertilised egg.

If pregnancy does not occur, the corpus luteum degenerates, progesterone levels fall, and the endometrium is shed as a menstrual period.

Norethisterone mimics the action of progesterone. When taken at sufficient doses, it maintains the endometrium in its secretory phase, preventing the hormonal withdrawal that triggers menstruation.

This is the basis for its use in period delay. At higher doses, norethisterone also suppresses the pituitary release of FSH and LH, inhibiting ovulation and follicular development.

This suppression, combined with its direct effects on the endometrium, underpins its use in treating endometriosis and heavy periods.

How norethisterone works: mechanism of action

Norethisterone is a 19-nortestosterone derivative with a strong affinity for the progesterone receptor. It exerts its clinical effects through several mechanisms.

First, it stabilises the endometrium by maintaining it in a secretory or atrophic state, preventing the breakdown and shedding that produce menstrual bleeding.

This is the primary mechanism for both period delay and the treatment of heavy periods.

Second, at high doses (10 to 25 mg daily), norethisterone suppresses the hypothalamic-pituitary-ovarian axis, inhibiting the release of gonadotrophins and preventing ovulation.

This is the basis for its use in endometriosis, where suppression of cyclical hormonal changes reduces the growth and activity of endometriotic tissue.

Third, norethisterone causes the cervical mucus to become thicker and less permeable to sperm, though this effect is more clinically relevant at the low contraceptive dose (350 micrograms) rather than the 5 mg therapeutic dose.

An important pharmacological consideration is that norethisterone is partly metabolised to ethinylestradiol (a synthetic oestrogen) in the body.

At doses of 10 mg or more per day, the amount of ethinylestradiol produced may be clinically significant, contributing to oestrogenic effects on clotting factors and potentially increasing the risk of venous thromboembolism.

This is an important safety consideration discussed in the warnings section below.

Clinical evidence and UK prescribing guidance

The use of norethisterone for period delay is well-established in clinical practice, though the evidence base consists largely of clinical experience and pharmacological rationale rather than large randomised trials.

The FSRH recognises norethisterone as an effective option for short-term delay of menstruation and notes that it is suitable for most women, with appropriate consideration of VTE risk at higher doses.

For heavy menstrual bleeding, NICE guideline NG88 recommends a stepwise approach: first-line pharmaceutical options include tranexamic acid (an antifibrinolytic), non-steroidal anti-inflammatory drugs (mefenamic acid, ibuprofen), combined oral contraceptives, cyclical progestogens (including norethisterone), and the levonorgestrel-releasing intrauterine system (Mirena).

The Mirena IUS is recommended as a first-line treatment if long-term management is required, as it is the most effective pharmaceutical option for reducing menstrual blood loss.

Norethisterone (cyclical or continuous) is an alternative for women who prefer oral treatment or who are not suitable for an IUS.

For endometriosis, NICE guideline NG73 recommends hormonal treatment options including combined oral contraceptives, progestogens (norethisterone, medroxyprogesterone acetate), the Mirena IUS, and GnRH analogues.

Norethisterone at doses of 10 to 25 mg daily is effective at suppressing endometriotic lesions and relieving pain but may cause side effects related to androgen activity and, at higher doses, oestrogenic VTE risk.

Norethisterone compared with other treatments

For period delay, norethisterone 5 mg three times daily is the standard oral option.

An alternative approach is to use a combined oral contraceptive pill without the usual 7-day break (running packs together), which is effective for women already taking the pill but is not suitable as a standalone period delay method for women not on hormonal contraception.

Medroxyprogesterone acetate is another progestogen that can be used for similar indications but is prescribed less frequently for period delay in the UK.

For heavy periods, the Mirena IUS reduces menstrual blood loss by up to 97% and is the most effective pharmaceutical treatment.

Tranexamic acid reduces blood loss by approximately 40 to 50% and is taken only during menstruation. Mefenamic acid and ibuprofen reduce blood loss by approximately 20 to 40%.

Combined oral contraceptives reduce blood loss by approximately 40 to 50%.

Cyclical norethisterone is effective but is generally considered a second-line option compared with the IUS and tranexamic acid.

For endometriosis, combined oral contraceptives (continuous use), the Mirena IUS, injectable progestogens (Depo-Provera), and GnRH analogues (goserelin) are alternatives to oral norethisterone.

The dienogest-containing preparation (Visanne) is a newer progestogen specifically licensed for endometriosis but is not currently available on the NHS in all areas.

Choice depends on severity, fertility wishes, tolerance of side effects, and patient preference.

Dosage and administration

For period delay, take 5 mg three times daily starting 3 days before the expected period, continuing for up to 10 to 14 days.

Your period will start 2 to 3 days after stopping.

For heavy menstrual bleeding, dosing varies by severity: 5 mg three times daily for 10 days is the standard regimen.

Cyclical prevention uses 5 mg twice daily from day 19 to 26 of the cycle.

For endometriosis, continuous dosing at 10 to 25 mg daily for 4 to 6 months is typical. Swallow tablets whole with water, with or without food.

Remember that norethisterone 5 mg is not a contraceptive; use reliable contraception if needed.

Side effects of norethisterone

Common side effects

The most commonly reported side effects are irregular bleeding or spotting, headache, nausea, breast tenderness, bloating, and mood changes.

Acne or oily skin may occur due to the mild androgenic properties of norethisterone.

These effects are usually mild and tend to resolve after stopping treatment or with dose adjustment.

Important safety considerations

At doses of 10 mg or more daily, norethisterone is partly metabolised to ethinylestradiol, which may increase the risk of venous thromboembolism (DVT and PE).

Symptoms of DVT include sudden leg swelling, pain, and redness. Symptoms of PE include sudden breathlessness, chest pain, and coughing up blood.

These are medical emergencies; call 999 immediately.

The risk with short-term period delay (5 mg three times daily for up to 14 days) is considered low but should still be assessed on an individual basis.

When to seek urgent medical advice

Stop norethisterone and seek urgent medical help if you develop sudden severe headache, visual disturbance, sudden leg pain or swelling, chest pain, breathlessness, or yellowing of the skin or eyes.

Call 999 in an emergency.

Contact your GP or call NHS 111 if you experience persistent mood changes, prolonged irregular bleeding, or signs of an allergic reaction (rash, swelling, difficulty breathing).

Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Contraindications

Norethisterone must not be used in women who are pregnant or suspected to be pregnant, have severe liver disease or active liver tumours, have undiagnosed vaginal bleeding, have known or suspected sex-steroid-dependent malignancies, have a current or past history of arterial thromboembolic disease, or have known hypersensitivity to the medicine.

VTE risk at high doses

Because norethisterone at 10 mg or more daily is partly metabolised to ethinylestradiol, prescribers should apply the same UKMEC eligibility criteria as for combined hormonal contraception.

Women with UKMEC category 3 or 4 conditions for combined hormonal contraception (including current or past VTE, known thrombophilia, BMI 35 or above, or migraine with aura) should not receive high-dose norethisterone unless the clinical benefit clearly outweighs the risk and no alternative is available.

Drug interactions

Enzyme-inducing medicines (rifampicin, carbamazepine, phenytoin, phenobarbital, St John's wort) may reduce the effectiveness of norethisterone. Norethisterone may affect the metabolism of ciclosporin and lamotrigine. Inform your prescriber of all medicines you are taking.

Pregnancy and breastfeeding

Norethisterone is contraindicated in pregnancy. If a period does not occur within 3 days of stopping the tablets, perform a pregnancy test.

Norethisterone is excreted in breast milk; short-term use for period delay is unlikely to have significant effects, but prolonged high-dose use during breastfeeding should be discussed with a prescriber.

How to get norethisterone in the UK

Norethisterone 5 mg tablets are a prescription-only medicine.

You can obtain a prescription from your GP, a sexual health clinic, or an authorised online prescriber registered with the GPhC.

The standard NHS prescription charge of 9.90 pounds applies in England; prescriptions are free in Scotland, Wales, and Northern Ireland.

Some online consultation services offer a streamlined process for period delay prescriptions.

Sources

• Norethisterone 5 mg Tablets, Summary of Product Characteristics (EMC)
• Norethisterone, British National Formulary (BNF)
• Heavy menstrual bleeding, NICE Guideline NG88
• Norethisterone, NHS
• MHRA Yellow Card Scheme