Changes to your periods are not a listed side effect of Mounjaro, Wegovy, Wegovy tablets or Foundayo in their UK product information. Periods can still change while you are on treatment, because weight change, stress, a new contraceptive, perimenopause and pregnancy can all affect your cycle. If your period is late or has stopped, take a pregnancy test first, and see your GP if your periods change.
This guide covers what to do if your period is late, how sickness can affect the pill, and which kinds of bleeding your GP should check. It applies to Mounjaro (tirzepatide), Wegovy injections and Wegovy tablets (semaglutide), and Foundayo (orforglipron). It is one of our guides to women's health on weight-loss treatment.
Can Mounjaro or Wegovy affect your periods?
We read the summary of product characteristics and the patient leaflet for all four medicines on 15 October 2026. None lists periods, missed periods, heavy periods or bleeding between periods as a side effect. The most common side effects include feeling sick, being sick, diarrhoea and constipation. Our guides to Mounjaro side effects and Wegovy side effects list the rest.
Where the product information mentions a cycle, it is in animal studies. Female rats given semaglutide, the medicine in Wegovy, had longer reproductive cycles and slightly fewer ovulations at doses that made them lose weight. For all four medicines, the product information says the effect on fertility in people is not known.
Orlistat, a weight loss medicine that works in a different way, is the exception. The product information for orlistat 120mg (Xenical) lists irregular periods as a common side effect, which means it may affect up to 1 in 10 people. The information for orlistat 60mg (alli) does not. Our orlistat page sets out its other side effects.
If you think a change in your periods could be linked to your medicine, tell us. You can also report it to the MHRA through the Yellow Card scheme.
What else can change your periods?
A typical cycle is around 28 days, and a period usually lasts about 5 days. The NHS describes periods as irregular if the gap between them is less than 21 days or more than 35 days. Common causes of irregular, late or missed periods on the NHS lists include:
- pregnancy: a missed period is often an early sign
- losing or gaining a lot of weight, sudden weight loss, and being overweight
- stress and anxiety
- exercising too much
- hormonal contraception, such as the pill, the progestogen-only pill, the contraceptive injection and the hormonal coil (IUS)
- the start of the menopause (perimenopause), usually between the ages of 45 and 55
- PMOS, which used to be called polycystic ovary syndrome (PCOS), and thyroid problems
Several of these can happen alongside treatment. You may be changing how you eat and move, or have changed your contraception because of Mounjaro or Foundayo, and the NHS says changes to your periods are common in the first few months of a new combined pill. In perimenopause, periods may come more or less often and get heavier or lighter. None of this means your medicine is the cause, or that a change should be ignored.
Is your period late on Mounjaro or Wegovy? Take a pregnancy test
Rule out pregnancy first. Most tests work from the first day of a missed period. If you do not know when your period was due, the NHS says to test at least 21 days after you last had unprotected sex. Taking the pill does not stop a test working. It is especially worth testing if you take the pill and missed the extra precautions for Mounjaro or Foundayo, or were sick or had diarrhoea around the time you took it.
If the test is positive, stop your weight loss medicine. The counselling we give every injection patient is:
Pregnancy: these medicines must not be used in pregnancy. Stop and contact us or your GP immediately if you become pregnant. If planning pregnancy: stop Mounjaro at least 1 month before, Wegovy at least 2 months before.
Wegovy tablets and Foundayo must not be used in pregnancy either. Whichever medicine you take, stop it and contact us or your GP straight away, and we will stop any further supply. Leaflets from the UK Teratology Information Service on medicines in pregnancy are on the Bumps website. Our guide to Mounjaro and pregnancy explains more.
If the test is negative and your period still has not come, test again in a few days. The NHS says to see your GP if a second test is negative and your period has still not arrived, or if you have missed 3 periods in a row.
Ask for an urgent GP appointment or contact NHS 111 if you have missed a period and have unusual vaginal bleeding with pain anywhere in your tummy. The NHS says this could be a sign of an ectopic pregnancy.
Can being sick or diarrhoea stop the pill working?
Sickness and diarrhoea are among the most common side effects of all four medicines, and they can mean less of your pill is absorbed. For the combined pill, the NHS advice is:
- If you are sick less than 3 hours after taking your pill (4 hours for Qlaira or Zoely), take another one straight away, then the next one at the usual time.
- If you keep being sick, use condoms as well until you have taken your pill for 7 days without being sick.
- If you have diarrhoea for more than 24 hours, keep taking your pill as normal and use condoms until 7 days after the diarrhoea stops.
For the progestogen-only pill, the NHS says it may not work if you are sick within 2 hours of a traditional progestogen-only pill or 3 to 4 hours of a desogestrel or drospirenone pill, or if you have severe watery diarrhoea. You may need emergency contraception, so speak to a pharmacist, a sexual health clinic or your GP, or call 111.
Two of the medicines can also make the pill less reliable even when you are well. For Mounjaro, the counselling we give is:
Contraception (Mounjaro only): tirzepatide can reduce absorption of the contraceptive pill. Use a non-oral method or add condoms for 4 weeks after starting and for 4 weeks after each dose increase. Wegovy needs no extra precautions unless severe vomiting or diarrhoea occurs, then follow missed-pill guidance.
For Foundayo, it is:
If you use the oral contraceptive pill, use a non-oral method or add a barrier method such as condoms for 30 days after starting treatment and for 30 days after each dose increase.
With Wegovy tablets, take your pill at least 30 minutes after the Wegovy tablet, as with any other medicine taken by mouth. Our guide to contraception and the pill on GLP-1 treatment covers each medicine in detail.
Heavy or unusual bleeding: when should you see your GP?
Do not assume that unusual bleeding is a side effect of your weight loss medicine. None of the four lists it, so get it checked as you would at any other time. The NHS says you may have heavy periods if you:
- need to change your pad or tampon every 1 to 2 hours, or use 2 types of period product together
- have periods lasting more than 7 days
- pass blood clots larger than about 2.5cm (the size of a 10p coin)
- bleed through to your clothes or bedding
- feel tired or short of breath a lot
See your GP if heavy periods are affecting your life or you have severe pain during your periods. Always get these checked by a doctor too:
- Bleeding between periods or after sex. The NHS says it is not usually serious, but to always get it checked by a GP or a sexual health clinic.
- Bleeding after the menopause, which means after 12 months without a period. The NHS says to see a GP even if it is a small amount or has only happened once.
On HRT and bleeding unexpectedly?
Some bleeding is expected when HRT is new. The NHS says irregular bleeding or spotting is common in the first few months and usually settles within 6 months, and a regular withdrawal bleed is normal on some types. Speak to your GP if irregular bleeding carries on for more than 6 months after starting HRT, becomes heavier, or starts after you have not had periods for a while.
None of the four medicines' product information mentions HRT. The British Menopause Society, in guidance for doctors, says the main concern is whether GLP-1 medicines, by slowing how quickly the stomach empties, reduce how much of a progestogen tablet is absorbed. Progestogen is the part of combined HRT that protects the lining of the womb. When that guidance was published in April 2025, there was no data on this. So tell whoever prescribes your HRT that you take a weight loss medicine, see your GP about any unexpected bleeding, and do not change or stop your HRT without their advice. Our guide to Mounjaro and HRT explains more.
What if you have PMOS (PCOS)?
PMOS (polyendocrine metabolic ovarian syndrome) is what the NHS now calls polycystic ovary syndrome (PCOS), and irregular periods or long gaps between them are one of its main symptoms. None of the medicines we supply is licensed to treat PMOS or to regulate periods, and their product information makes no claim about either. Your GP remains the place to discuss treatment for PMOS. Our guide to Mounjaro and PCOS has more.
Irregular periods do not mean you cannot become pregnant, so keep using contraception throughout treatment, as the MHRA advises. If you are planning a pregnancy, see our guide to Mounjaro and fertility.
What should you note about your cycle on treatment?
The NHS suggests using an app or a diary to keep track of your periods, and keeping a diary of your symptoms before you see your GP about period problems. On treatment, it helps to note:
- the first day of each period and how many days it lasts
- how heavy it is, and any clots larger than a 10p coin
- any bleeding between periods or after sex, and any pain
- the date you started treatment and the date of each dose increase
- any sickness or diarrhoea, missed pills or change of contraception
- the date and outcome of any pregnancy test
Take the note to your GP, so they can see what changed and when.
How we check before we supply
Our consultation asks whether you are pregnant, breastfeeding or actively trying to conceive, and we cannot supply weight loss medicine if you are. It also asks whether you take the contraceptive pill, and every reorder asks again whether you are pregnant, planning a pregnancy or breastfeeding. A GPhC-registered pharmacist reviews every consultation, and treatment decisions are made with our pharmacist.
If your periods change and you are not sure what to do, contact us. Your GP can also tell you about NHS weight management services. To see whether treatment may be suitable for you, check if you are eligible.