Part of our complete guide to periods in the UK.
Period Delay: The Facts — How It Works & Who Can Use It
A UK clinical guide to the science of period delay — how norethisterone works at the hormonal level, the evidence base, contraindications, and who can use it safely.
▶ How does period delay work?
Period delay works through progesterone withdrawal prevention. Norethisterone 5mg is a synthetic progestogen that mimics natural progesterone — maintaining elevated levels in the luteal phase and preventing the endometrial shedding signal. The period is postponed for as long as the tablets are taken, then arrives 2–3 days after stopping.
Norethisterone is one of the most widely prescribed period delay medications in the UK — yet the science behind how it works is rarely explained in detail. This guide covers the clinical mechanism of period delay: what actually happens when you take norethisterone, the evidence base behind it, who can safely use it, and who cannot. For the practical question of whether to delay your period and how to plan it, see: Can You Delay Your Period? — UK Planning Guide →
Get Norethisterone Online
GPhC-registered pharmacist independent prescribers. Order before 3pm Monday–Friday for next-day delivery. GPhC pharmacy #9011198.
View Period Delay Treatment →The Menstrual Cycle and Why Periods Happen
Understanding why periods occur is the foundation for understanding how norethisterone prevents them. The menstrual cycle is driven by four hormones — oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH) — cycling through four distinct phases:
Menstruation (Days 1–5)
Progesterone and oestrogen fall. The uterine lining sheds as a period. FSH begins rising to stimulate follicle development.
Follicular phase (Days 1–13)
Oestrogen rises as follicles develop. The uterine lining rebuilds and thickens.
Ovulation (Day ~14)
LH surge triggers ovulation. The dominant follicle releases an egg.
Luteal phase (Days 15–28)
Progesterone rises from the corpus luteum to maintain the lining. If no pregnancy, progesterone falls — triggering menstruation.
The progesterone withdrawal at the end of the luteal phase is the direct cause of menstruation. Norethisterone's mechanism of action targets this specific trigger.
How Norethisterone Delays a Period: The Mechanism
Norethisterone (norethindrone) is a synthetic progestogen — a man-made molecule designed to bind to progesterone receptors and mimic the biological effects of natural progesterone. When taken orally, it:
- Maintains elevated progesterone receptor signalling at the endometrium — preventing the withdrawal signal that triggers shedding
- Suppresses FSH and LH secretion at the doses used, potentially affecting follicular development and ovulation during the treatment period
- Thickens cervical mucus — a secondary effect relevant at higher contraceptive doses; less clinically significant at the 5mg period delay dose
- Maintains the endometrium in a secretory state, stable and unshed, for as long as tablets continue
Once norethisterone is stopped, circulating progestogen levels fall within hours to days. The endometrium receives the withdrawal signal and begins to shed — producing the "withdrawal bleed" that arrives 2–3 days after the last tablet.
Not the same as pregnancy: Norethisterone maintains the uterine lining through the same signalling mechanism that progesterone uses in early pregnancy — but does not create or sustain a pregnancy. It is simply preventing the withdrawal signal that would otherwise trigger your period.
What the Evidence Shows
17
Maximum days of delay typically achievable with norethisterone
3
Minimum days before expected period to start for reliable effectiveness
~1 in 10
Women experience light spotting — not treatment failure
Decades
Of use in UK clinical practice with established safety profile
Norethisterone has been licensed and used in the UK for decades. The MHRA and NICE support its short-term use for period delay at the 5mg three-times-daily dosing schedule. Clinical evidence confirms that starting at least 3 days before the expected period onset is required for reliable effectiveness — starting later allows the withdrawal process to begin before progesterone levels can be maintained.
Approximately 1 in 10 women experience breakthrough bleeding or light spotting while taking norethisterone. This is not treatment failure — it reflects individual variation in progesterone receptor sensitivity and endometrial stability. Heavy bleeding while taking norethisterone warrants prescriber contact.
Who Can and Cannot Use Norethisterone
Norethisterone is suitable for most healthy women of reproductive age, but a clinical assessment must screen for the following contraindications:
| Contraindication | Clinical reason |
|---|---|
| Pregnancy | Synthetic progestogens are not appropriate during pregnancy. A pregnancy test may be required before prescribing. |
| History of venous thromboembolism (DVT or PE) | Progestogens increase clotting risk in predisposed individuals. Prior VTE is a significant contraindication. |
| Severe liver disease or liver tumours | Norethisterone is hepatically metabolised; impaired liver function affects both efficacy and safety. |
| Hormone-sensitive cancers | Including oestrogen- and progesterone-receptor-positive breast cancers. Progestogen stimulation may promote growth. |
| Unexplained vaginal bleeding | The underlying cause must be investigated before starting any hormonal treatment that could mask symptoms. |
| Breastfeeding | Norethisterone passes into breast milk; alternatives should be discussed. |
| Severe arterial or cardiovascular disease | Including severe hypertension or arterial disease where progestogen risk requires careful weighing. |
Caution — not regular-use medication: Norethisterone for period delay is intended for occasional use, not as a monthly solution for cycle management. Regular monthly use increases cumulative progestogen exposure and side effect risk. Women requiring long-term cycle control should discuss this with their prescriber — the combined pill, progesterone-only pill, or hormonal IUS may be more appropriate.
Other Clinical Uses of Norethisterone
At different doses and schedules, norethisterone is also prescribed for:
- Heavy periods (menorrhagia) — typically day 5 to day 26 of the cycle at higher dose
- Endometriosis — continuous higher-dose norethisterone suppresses endometrial tissue growth
- Premenstrual syndrome (PMS) — luteal phase dosing
- As a component of combined oral contraceptive pills
These uses involve different doses, durations, and clinical oversight from period delay. Period delay at 5mg three times daily does not provide contraceptive cover.
Ready to Delay Your Period?
For the practical planning guide — how far in advance to order, what to expect, and how to get a prescription online — see: Can You Delay Your Period? UK Planning Guide →
Get Norethisterone Online
GPhC-registered pharmacist independent prescribers. GPhC pharmacy #9011198. Discreet next-day delivery.
View Period Delay Treatment →Frequently Asked Questions
How does norethisterone delay a period?
Norethisterone delays a period by mimicking progesterone in the second half of the menstrual cycle. Normally, progesterone levels fall at the end of the luteal phase, signalling the uterine lining (endometrium) to shed. Norethisterone maintains artificially elevated progesterone levels, preventing this withdrawal signal. The lining stays in place for as long as the tablets are taken. When norethisterone is stopped, levels fall and the period arrives within 2–3 days.
Is it safe to delay your period with norethisterone?
Yes, for most healthy women when used short-term as prescribed. Norethisterone has been used in the UK for decades. Clinical evidence and MHRA guidance support its short-term use for period delay. It is not intended for regular monthly use. A clinical consultation screens for contraindications before prescribing.
How effective is norethisterone for period delay?
When started at least 3 days before the expected period and taken correctly (5mg three times daily), norethisterone successfully postpones menstruation in the vast majority of women. Around 1 in 10 women experience light spotting or breakthrough bleeding — this does not mean treatment has failed.
Does norethisterone affect fertility long-term?
No long-term effects on fertility have been established. Normal menstrual cycles and fertility return shortly after stopping norethisterone. Short-term period delay with norethisterone does not affect future fertility.
What conditions make norethisterone unsafe?
Norethisterone is contraindicated in women with a personal history of blood clots (DVT or PE), severe liver disease, hormone-sensitive cancers (oestrogen- or progesterone-receptor-positive), unexplained vaginal bleeding, pregnancy, or breastfeeding. A prescriber reviews medical history before any prescription is issued.
What other conditions is norethisterone used for?
At different doses, norethisterone is also prescribed for heavy periods (menorrhagia), painful periods (dysmenorrhoea), endometriosis, premenstrual syndrome, and as a component of some combined oral contraceptive pills. These uses are at different doses and schedules from period delay.
References
- MHRA. SPC: Norethisterone 5mg tablets. 2024. medicines.org.uk/emc
- NICE. Heavy menstrual bleeding: assessment and management (NG88). Updated 2023. nice.org.uk
- NICE. Norethisterone: CKS prescribing information. 2024. cks.nice.org.uk
- NHS. Norethisterone. nhs.uk/medicines/norethisterone
- Shakespeare J et al. Is norethisterone a lifestyle drug? BMJ. 2000;320:291.
Medical disclaimer: This article is for informational purposes only. Norethisterone is a prescription-only medicine in the UK. A clinical consultation is required before it can be dispensed. In a medical emergency, call 999.