Mounjaro and Period Changes: What to Know

Last updated: 18 September, 2026

Have you noticed your period is different since starting Mounjaro? Some people have reported menstrual cycle changes while losing weight. These changes can include irregular periods or a change in flow.

Mounjaro (tirzepatide) acts like two hormones that your body makes naturally:

GLP-1 (glucagon-like peptide-1)
GIP (glucose-dependent insulinotropic polypeptide)

This helps regulate blood sugar levels, improve insulin sensitivity, and reduce your appetite.

Because Mounjaro affects hormones, you might wonder if this impacts other aspects of hormonal health, including the menstrual cycle. Or whether period changes are one of the Mounjaro side effects.

Can Mounjaro change your menstrual cycle?

Research has not confirmed that Mounjaro directly affects the menstrual cycle. Mounjaro’s patient information leaflet does not list period changes as a side effect.

“Some users do report changes to their cycle,” says Abdal Alvi, Simple Online Pharmacy’s Chief Clinical Officer. “But the evidence is currently limited.”

In the US, patients and healthcare professionals can report suspected side effects to a national database. A 2026 study of these reports found that, compared with other medicines, tirzepatide had a higher share of reports for two problems. These were bleeding between periods and clots in period blood. The study did not find the same pattern for heavy bleeding or missed periods. These reports cannot prove Mounjaro caused the changes or show how common they are.

Mounjaro (tirzepatide) does affect hormones, and it helps your body respond better to insulin. Insulin also plays a part in how the ovaries work. So it is possible that these hormone changes could affect your menstrual cycle too.

If your cycle has changed on Mounjaro, your weight loss may be one reason. Research has found weight and menstruation to be linked. The NHS lists losing or gaining a lot of weight as a common cause of irregular cycles. Weight changes can affect your levels of oestrogen, a hormone that regulates the menstrual cycle.

Healthy weight loss may help regulate your periods. A review of 11 clinical trials showed GLP-1 weight loss treatment contributed to more regular periods in people with PCOS carrying extra weight. PCOS stands for polycystic ovary syndrome, which the NHS now calls polyendocrine metabolic ovarian syndrome (PMOS).

Can Mounjaro stop periods?

There’s no evidence that Mounjaro stops periods, but rapid weight loss can.

If your period has stopped, don’t ignore it. Take a pregnancy test first, because pregnancy is one of the most common reasons a period stops. It could also be a sign your weight loss has been too quick, or that you’re eating too little on Mounjaro. Significant weight loss stresses the body. In response, your body prioritises your essential functions over menstruation and producing hormones like oestrogen.

Once you’re eating enough and your weight settles, periods may take 6 to 12 months or longer to return. You can release an egg before your first period comes back, so pregnancy is still possible.

If you’ve missed three periods in a row, talk to your GP. You should also check in with us or your GP if you notice any of the following:

Your cycles are a lot shorter or longer than usual
You have very heavy or very light periods
You’re experiencing significant changes in premenstrual symptoms
You’re getting severe cramping or pain is getting much worse

Your GP or our prescribers can determine if Mounjaro, or another factor, is causing these changes.

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How can weight loss affect your menstrual cycle?

Losing weight too fast and being underweight can lower your oestrogen levels. This may cause irregular periods, and they could even stop completely.

However, being overweight can raise your oestrogen levels, because body fat turns other hormones into oestrogen. Too much oestrogen can stop an egg from being released, which can make periods irregular. Carrying extra weight has also been linked to heavier periods, although the reasons are not fully clear.

If you’re living with overweight or obesity, losing weight may help rebalance these hormones. You might experience some of the following:

Regular cycles: A healthy body mass index (BMI) may bring your cycle closer to a regular pattern. The NHS considers a gap of 21 to 35 days between periods to be regular.
Lighter periods: Better hormone regulation through weight loss may reduce your flow, although the evidence is limited.
Less cramping: People with a higher BMI report more stomach cramps before their period. Whether losing weight eases cramps is less clear.
Improved PMS symptoms: Premenstrual syndrome (PMS) can cause mood swings, irritability, low mood and anxiety. In a large US study, each one-point rise in BMI was linked to a 3% higher risk of developing PMS. The study did not test whether losing weight eases symptoms.

A healthier diet, regular exercise and better sleep habits may also help. NHS inform suggests gentle exercise, balanced eating and 7 to 8 hours of sleep a night to ease PMS symptoms.

Your hair can also change while you lose weight on Mounjaro. Mounjaro hair loss affected about 1 in 20 people in the weight loss trials. Around 1 in 100 people taking a placebo, an inactive injection given for comparison, also lost hair. Most cases were mild, and people usually recovered without stopping treatment.

Monitoring menstrual changes on Mounjaro

If you’re concerned about Mounjaro and period changes, try tracking your menstrual cycle. Tracking can help you understand how the medication might be affecting your body.

Documenting the following details can provide insights for yourself, your GP and our prescribers:

Your cycle length (the first day of one period to the first day of the next)
Your flow
If you’re bleeding between periods
Any clots in your period blood, and roughly how big they are
Any symptoms, including cramping, mood changes, fatigue, bloating and headaches
The date of each Mounjaro injection and any dose increase

Let us or your GP know about any changes that worry you.

“What we always advise is that a person reports any changes to their cycle onto their healthcare professional,” Abdal says. “If a person thinks that they might be pregnant…[they should] reach out to us or our Patient Care team or their own GP.”

If you’re a patient with Simple, we’re here for you throughout your weight loss journey. If you have any questions or concerns, get in touch with our Patient Care team on 0141 774 5023.

Can Mounjaro make the contraceptive pill less effective?

Mounjaro may make the contraceptive pill less effective. Mounjaro slows stomach emptying, which can reduce how much of the pill your body absorbs. The effect is strongest when you first start treatment.

The Medicines and Healthcare products Regulatory Agency (MHRA) is the UK’s medicines regulator. If you take the pill while on Mounjaro, the MHRA advises also using a second method, such as condoms. This matters most for four weeks after you start Mounjaro, and for four weeks after every dose increase, including the one that takes you to your Mounjaro maintenance dose.

Alternatively, you could switch to a non-oral method of contraception, such as the implant, the injection or a coil. Mounjaro affects how well the pill is absorbed from the gut, so methods that do not rely on absorption from the gut are not affected in the same way and do not need the extra cover.

Sickness and diarrhoea can also stop the pill from working, and both are common side effects of Mounjaro. If you vomit within three hours of taking the combined pill, the NHS advises taking another pill straight away. Diarrhoea lasting more than 24 hours means using condoms until seven days after it stops.

What should you do if you might be pregnant?

Take a pregnancy test and contact us straight away if you think you might be pregnant. The MHRA advises stopping GLP-1 medicines such as Mounjaro as soon as possible if you get pregnant. Researchers do not yet have enough safety data to know whether these medicines could harm a baby.

Data from 11 clinical trials showed older GLP-1 medicines raised natural pregnancy rates in people with PCOS who were carrying extra weight. Mounjaro takes time to leave your body. If you’re planning a baby, talk to us about stopping Mounjaro at least a month before you start trying.

When should I see a doctor?

Most period changes are not an emergency, but some symptoms need medical help quickly.

Contact our clinical team if:

You take the pill and are starting Mounjaro or moving up a dose
You are not sure whether your contraception is still reliable
Sickness or diarrhoea keeps coming back but is manageable
Your cycle has changed, and you want to talk through your treatment

See your GP or call NHS 111 if:

You have missed three periods in a row and a pregnancy test is negative
You bleed between periods or after sex
Your periods have started lasting longer than seven days, when they usually don’t
Your periods become much heavier, or you pass clots larger than a 10p coin
You get a positive pregnancy test: stop Mounjaro and tell us as well
You might be pregnant and have pain low down on one side of your stomach, shoulder tip pain or unusual bleeding
You have pain in the upper right of your stomach that lasts longer than 30 minutes
Vomiting or diarrhoea stops you keeping fluids down for more than six hours

Call 999 or go to A&E if you notice any of the following:

Severe stomach pain that spreads to your back and does not go away: stop taking Mounjaro
Swelling of your face, lips, tongue or throat, or difficulty breathing
Sudden, sharp and intense stomach pain with dizziness, fainting or feeling sick if you might be pregnant
Your skin or the whites of your eyes turn yellow
Signs of severe dehydration after repeated vomiting or diarrhoea

Whatever the cause, if you have thoughts of harming yourself, go to A&E and ask for the CRISIS team.

Key takeaways

Period changes aren’t a listed side effect of Mounjaro. Reports of bleeding between periods and clots in period blood were higher than expected for tirzepatide in a 2026 study. This does not prove Mounjaro causes them.
Weight loss may improve cycle regularity. Studies of older GLP-1 medicines suggest weight loss treatment could lead to more regular cycles in people with PCOS. This may be particularly true if you experience irregular periods from weight-related hormonal imbalances.
Irregular periods and stopped periods may signal you’re losing weight too fast. Take a pregnancy test first, then let us or your GP know if you’ve noticed unusual changes to your menstrual cycle.
A higher BMI is linked to PMS, but the benefit of losing weight is less clear.
Monitor changes and discuss them with us. If you notice any significant changes in your menstrual cycle, like bleeding between periods, keep a note of changes. If you take the pill, ask us whether you need extra contraception when you start Mounjaro or increase your dose.