Norethisterone is generally safe for short-term use in healthy women. It’s a synthetic progestogen prescribed across the UK and many other countries for period delay, heavy menstrual bleeding, endometriosis, and contraception. Like all hormonal medications, it carries some risks that depend on your health history, the dose you’re taking, and how long you use it.
What Norethisterone Is Used For
Norethisterone has several distinct uses, and the safety profile shifts depending on which one applies to you. At low doses (around 350 micrograms daily), it works as a progestogen-only contraceptive pill. At higher doses (5 mg, taken three times daily), it’s commonly used for short-term period delay, typically for holidays, events, or religious observances. Doses of 5 to 15 mg daily are also used therapeutically for heavy periods, endometriosis pain, and abnormal uterine bleeding.
The short-term period delay use is probably the most common reason people search for safety information. For this purpose, you typically start taking it three days before your expected period and continue for as long as you need to delay it. Your period arrives a few days after you stop. Research has found norethisterone more effective than combined oral contraceptive pills at delaying menstruation and preventing breakthrough bleeding, with higher patient satisfaction rates.
Common Side Effects
Most side effects are mild and resolve once you stop taking the medication. The ones reported most frequently include headaches, nausea, breast tenderness, bloating, mood changes, and spotting or breakthrough bleeding. These are typical of progestogen-based hormones generally, not unique to norethisterone.
Some women notice changes in skin (acne), appetite, or libido. At higher therapeutic doses used for conditions like endometriosis, side effects tend to be more pronounced because you’re taking the medication for longer and at greater concentrations. Weight changes and fluid retention are more commonly reported in longer-term users.
Blood Clot Risk
Blood clots are the safety concern that worries most people, and the evidence here is reassuring. A large pooled analysis covering over 146,000 woman-years of observation compared norethisterone users to levonorgestrel users (levonorgestrel is considered the lowest-risk progestogen for clots). The clot rate for norethisterone users was 6.9 per 10,000 woman-years, compared to 8.8 per 10,000 for levonorgestrel users. After statistical adjustment, the researchers could confidently rule out even a 1.5-fold increased risk from norethisterone relative to levonorgestrel.
That said, these numbers apply to combined formulations with estrogen. Progestogen-only norethisterone at the low contraceptive dose carries an even lower clot risk because estrogen is the main driver of clotting changes. The higher doses used for period delay (5 mg three times daily) are partially converted to estrogen in the body, which is why some prescribers treat the clot risk of high-dose norethisterone more cautiously, closer to that of a combined pill.
Who Should Not Take It
Norethisterone is not suitable for everyone. You should not use it if you have:
- Active or past breast cancer
- Liver disease, liver tumors, or liver cancer
- Unexplained vaginal bleeding that hasn’t been investigated
- A history of blood clots such as deep vein thrombosis or pulmonary embolism (this applies specifically to norethisterone acetate tablets)
- A history of heart attack or stroke
If you have risk factors for clots but no personal history (for example, you smoke, have a high BMI, or are over 35), the decision depends on the dose and duration. A few days of low-dose progestogen-only norethisterone is very different from weeks of high-dose treatment, and your prescriber will weigh these factors.
Bone Density With Long-Term Use
Injectable forms of norethisterone (norethisterone enanthate), used as long-acting contraception, have been linked to reduced bone mineral density. Research on young women aged 19 to 24 found significantly lower bone density at the spine, hip, and femoral neck in those who used injectable progestogens exclusively for four to five years, compared to non-users. This effect was not seen in women who alternated between injectables and combined oral contraceptive pills.
This concern primarily applies to the injectable form, not to short courses of oral tablets. Regulatory agencies have cautioned against using injectable progestogen contraceptives for more than two years in young women because of this bone density effect. If you’re taking oral norethisterone for a few days to delay a period, bone health is not a relevant concern. For longer therapeutic courses (treating endometriosis, for example), total treatment duration is typically capped at 12 months partly because of bone density considerations.
Safety During Breastfeeding
Low-dose norethisterone (350 micrograms daily) used as a contraceptive is considered compatible with breastfeeding, though nonhormonal methods are generally preferred. Studies have measured very small amounts of the drug in breast milk, peaking at two to four hours after a dose and declining throughout the day. Research tracking infants whose mothers used progestogen-only contraceptives has found no consistent differences in physical or mental development.
The picture on milk supply is mixed. Most studies show no negative effect or even a slight increase in milk volume and duration of breastfeeding. One small study did find reduced milk quantity and lower protein, lipid, and calcium content compared to mothers using nonhormonal contraception. No data exists on the safety of high-dose norethisterone acetate (the kind used for period delay or endometriosis) during breastfeeding, so that use is generally avoided.
Drug Interactions to Be Aware Of
Norethisterone is broken down by liver enzymes, which means medications that speed up liver metabolism can reduce its effectiveness. The most relevant examples include certain anti-seizure medications, some antibiotics (particularly rifampicin), and St. John’s Wort. Medications that slow down these liver enzymes can increase norethisterone levels in your blood, potentially intensifying side effects. If you take regular medication, it’s worth checking for interactions before starting norethisterone.
Short-Term Use for Period Delay
For the most common scenario, taking norethisterone for a week or two to shift the timing of a period, the safety profile is favorable for healthy women without the contraindications listed above. Side effects are usually tolerable and temporary. The clot risk, while slightly elevated compared to taking nothing at all, is small in absolute terms and comparable to what you’d experience from a standard combined contraceptive pill. Most women who have used it for event-related period delay report high satisfaction, with about 80% saying they’d choose the method again in one study of women delaying bleeding before their weddings.