Mounjaro and periods: menstrual changes on treatment

In short

Period changes are not listed in the Mounjaro SmPC or patient leaflet
The NHS lists being overweight and sudden weight loss among causes of missed or late periods
Pill users should add a barrier method or switch to a non-oral method for 4 weeks after starting and 4 weeks after each dose increase
Stop Mounjaro at least 1 month before trying to get pregnant
See a GP if you miss 3 periods in a row, bleed between periods or after sex, or bleed after the menopause

Mounjaro is not known to act on the menstrual cycle directly, and period changes are not listed as a side effect in the UK leaflet. Weight change does affect periods, and the NHS lists both being overweight and sudden weight loss as causes of missed or late periods. As your weight falls, your cycle may become more regular, less regular, lighter or heavier. Losing weight may also make ovulation more regular, so you need reliable contraception.

What the official information says

The Summary of Product Characteristics (SmPC) for Mounjaro does not list changes to periods, missed periods or heavier bleeding as adverse reactions. The patient leaflet does not mention periods.

The SmPC does cover three related points:

Pregnancy. Tirzepatide should not be used in pregnancy and is not recommended in women of childbearing potential not using contraception (section 4.6).
Planning a pregnancy. Stop at least 1 month before a planned pregnancy (section 4.6). The MHRA gives the same figure for Mounjaro.
Fertility. The effect of tirzepatide on human fertility is unknown. Animal studies showed no direct harm (section 4.6).

How weight change affects your cycle

Your periods depend on a balance of hormones from the brain and ovaries. Body fat produces oestrogen and affects insulin levels, so a large change in body fat can shift the balance.

The NHS says being overweight or obese (a BMI of 30 or over) reduces fertility, and excess weight can affect ovulation. It also lists sudden weight loss, stress and being overweight among the causes of missed or late periods. On Mounjaro, both pressures can apply at once: a lower weight may help ovulation settle, while fast loss and eating little may unsettle it.

What people commonly notice:

Change	Possible reason
Periods return or become regular after months of gaps	Ovulation becoming more regular as weight falls
A late or missed period	Fast weight loss, low food intake, stress or pregnancy
Heavier or longer bleeds	Hormone shifts as body fat changes, or a separate cause such as fibroids
Lighter periods	Lower body fat, or hormonal contraception
More period pain or a new pattern of symptoms	Return of ovulation in someone who rarely ovulated before

None of these have been measured in the tirzepatide trials. The table lists likely reasons, not proven effects of the medicine.

PCOS and PMOS

Polycystic ovary syndrome (PCOS) is now called polyendocrine metabolic ovarian syndrome (PMOS) by the NHS. Irregular periods or long gaps between periods are one of its main symptoms. The NHS advises people with PMOS who are overweight to try to lose weight and lists weight loss medicines among the treatments.

Mounjaro is not licensed to treat PMOS. Some people with PMOS find their periods come back or become regular as they lose weight. If you have PMOS and have relied on irregular cycles as a sign of low fertility, take extra care with contraception.

Fertility and the contraception point

Because weight loss may make ovulation more regular, some people become pregnant on Mounjaro without planning to. Two things raise the risk:

Ovulation returning after years of irregular cycles
The pill working less well on Mounjaro

The pill and Mounjaro

The SmPC reports lower levels of oral contraceptive hormones after a dose of tirzepatide. The MHRA says Mounjaro may reduce the effectiveness of oral contraceptives in people who are overweight.

The advice from the SmPC, the MHRA and the contraception guidance from the Faculty of Sexual and Reproductive Healthcare (FSRH, now the College of Sexual and Reproductive Healthcare) is the same. If you take an oral contraceptive:

Add a barrier method such as a condom, or switch to a non-oral method, for 4 weeks after starting Mounjaro
Do the same for 4 weeks after each dose increase

The FSRH guidance suggests non-oral methods such as the implant, the coil (IUD or IUS), the vaginal ring or the patch. These do not rely on absorption from the gut. The contraceptive pill page goes through each option.

Vomiting and diarrhoea

Sickness and diarrhoea are common in the early weeks. The FSRH guidance says to follow the missed pill rules if you vomit within a few hours of taking a pill or have severe diarrhoea for more than 24 hours. If vomiting or diarrhoea keeps happening, consider a non-oral method and use condoms consistently. The nausea and vomiting page has ways to reduce sickness.

If you want to get pregnant

Stop Mounjaro at least 1 month before you start trying. If you find you are pregnant while using it, the MHRA advises you to stop the medicine as soon as possible and speak to your healthcare professional. More detail is on the pregnancy page.

Heavy periods and iron

The NHS describes heavy periods as needing to change a pad or tampon every 1 to 2 hours, periods lasting more than 7 days, or passing clots larger than about 2.5cm (the size of a 10p coin). Heavy bleeding drains iron. On Mounjaro you may eat less red meat, pulses and fortified foods than before, so iron stores can fall faster. Tiredness, breathlessness and hair shedding are common signs. See iron, B12 and vitamin D for food sources and when to ask for a blood test.

Perimenopause, menopause and HRT

Periods often become irregular in your 40s as you approach the menopause, whether or not you take Mounjaro. If you use HRT, tell your prescriber about any new bleeding pattern. Any bleeding after the menopause needs a GP check. The Mounjaro and HRT guide covers this in more detail.

When to get help

Call 999 or go to A&E if you have:

Heavy bleeding with dizziness, fainting, a racing heart or breathlessness
Sudden severe pain low in your tummy, especially if your period is late or you might be pregnant (possible ectopic pregnancy)

Call 111 or see a GP today if:

You have a positive pregnancy test while using Mounjaro
You are soaking through a pad or tampon every hour for several hours
You have a high temperature with unusual discharge or pelvic pain

Mention at your next check if:

You have missed your period 3 times in a row, after a negative pregnancy test
Your periods have become irregular, heavier or more painful
You want to switch from the pill to a non-oral method of contraception
You feel tired and breathless and want an iron test

Bleeding between periods, after sex or after the menopause needs a GP appointment in the next few days rather than waiting for a routine check. Report any side effect to the MHRA through the Yellow Card scheme.

Questions people ask

Can Mounjaro affect your periods?

Period changes are not listed as a side effect of Mounjaro in the UK leaflet. Weight change affects periods, though, and Mounjaro causes large weight loss. The NHS lists both being overweight and sudden weight loss as causes of missed or late periods, so cycles can become more regular, less regular, lighter or heavier while your weight is changing.

Can Mounjaro make you more fertile?

Mounjaro has no known direct effect on fertility, and the SmPC says the effect on human fertility is unknown. The NHS says a BMI of 30 or over reduces fertility and excess weight can affect ovulation. Losing weight may make ovulation more regular, so pregnancy may become more likely than before. Use effective contraception.

Does Mounjaro stop the pill working?

Mounjaro may make the pill less effective in people who are overweight, because it changes how the stomach empties. The SmPC and the MHRA advise adding a barrier method such as a condom, or switching to a non-oral method, for 4 weeks after starting and for 4 weeks after each dose increase. The coil, implant, patch and ring do not depend on absorption from the gut.

My period is late on Mounjaro. What should I do?

Take a pregnancy test first, especially if you use the pill and have had vomiting or diarrhoea. If the test is positive, stop Mounjaro and speak to your prescriber or GP as soon as possible. If it is negative, a single late period during fast weight loss is common, but see a GP if you miss 3 in a row.

Can Mounjaro help PCOS?

Mounjaro is not licensed to treat PCOS, which the NHS now calls polyendocrine metabolic ovarian syndrome (PMOS). The NHS advises people with PMOS who are overweight to try to lose weight and lists weight loss medicines among the treatments. Some people with PMOS find their periods become more regular as they lose weight.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro and periods: menstrual changes on treatment. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-and-periods/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Changes to periods are not listed as a side effect in the UK leaflet for Mounjaro. Weight change itself affects the menstrual cycle, and the NHS lists both being overweight and sudden weight loss as causes of missed periods. Losing weight may make ovulation more regular and pregnancy more likely than before, so effective contraception matters. If you take the pill, add a barrier method or switch to a non-oral method for 4 weeks after starting and after each dose increase.

Period changes are not listed in the Mounjaro SmPC or patient leaflet
The NHS lists being overweight and sudden weight loss among causes of missed or late periods
Pill users should add a barrier method or switch to a non-oral method for 4 weeks after starting and 4 weeks after each dose increase
Stop Mounjaro at least 1 month before trying to get pregnant
See a GP if you miss 3 periods in a row, bleed between periods or after sex, or bleed after the menopause

Cite as: Mounjaro Info, "Mounjaro and periods: menstrual changes on treatment", https://mounjaroinfo.co.uk/side-effects/mounjaro-and-periods/, reviewed 22 October 2026.