The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Does Mounjaro Cause Heavy Periods?
Mounjaro (tirzepatide) is a prescription-only weight management injection that may affect your menstrual cycle, though heavy periods are not listed as a common side effect in UK clinical data. Some women taking GLP-1 and GIP/GLP-1 medications report changes to their period — including heavier, lighter, or irregular bleeding — but individual experiences vary widely. If you notice significant changes to your menstrual cycle after starting Mounjaro, it's important to discuss this with your GP. They can assess whether the changes are related to the medication, weight loss itself, or another factor.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
What Does the Clinical Evidence Say?
Mounjaro's UK clinical trials (SURMOUNT-1 and related studies) did not specifically highlight heavy menstrual bleeding as a frequent or serious side effect. However, menstrual cycle changes are occasionally reported by women using GLP-1 and dual GIP/GLP-1 medications in real-world use. These changes may include irregular periods, heavier bleeding, lighter bleeding, or missed periods.
Weight loss itself can affect hormonal balance and menstrual cycles. gradual weight management or significant body composition changes may influence oestrogen levels, which regulate menstruation. Additionally, tirzepatide affects how quickly food moves through your stomach and can influence nutrient absorption — if you're not absorbing nutrients properly, this too may impact your cycle. It's difficult to isolate whether changes are caused directly by Mounjaro, the weight loss process, or underlying health factors.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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When Should You Contact Your GP?
Contact your GP if you experience severe or prolonged heavy bleeding, bleeding lasting more than 7 days, or if menstrual changes are causing you concern or affecting your quality of life. Your GP can:
- Rule out other causes (such as polyps, fibroids, or thyroid issues)
- Check your iron levels (heavy bleeding can lead to anaemia)
- Assess whether the change is related to Mounjaro, weight loss, or another factor
- Discuss whether continuing Mounjaro is appropriate for you
- Suggest management options if changes persist
Do not stop taking Mounjaro without guidance from your prescriber — stopping suddenly is not recommended.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is heavy menstrual bleeding a known side effect of Mounjaro in the UK?
Heavy periods are not listed as a common or very common side effect of Mounjaro in UK clinical trials. However, some women report menstrual changes (heavier, lighter, or irregular bleeding) when using GLP-1 and GIP/GLP-1 medications. These changes may be related to weight loss, hormonal shifts, or the medication itself. Individual experiences vary.
Can weight loss from Mounjaro affect my period?
Yes. Significant weight loss and rapid body composition changes can influence oestrogen levels and hormone balance, which regulate menstruation. This may cause your period to become heavier, lighter, irregular, or temporarily stop. This is separate from any direct effect of tirzepatide and is a known effect of gradual weight management generally.
What if my periods are heavier after starting Mounjaro?
If you notice heavier periods after starting Mounjaro, contact your GP. They can assess whether the change is related to the medication, weight loss, or another cause (such as thyroid issues or polyps). Your GP may check your iron levels, as heavy bleeding can lead to anaemia. Do not stop Mounjaro without medical guidance.
Does Mounjaro interact with the contraceptive pill and affect my period?
Mounjaro may reduce how well your body absorbs oral medications, including the contraceptive pill. The Patient Information Leaflet recommends using additional barrier contraception for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. This may affect your period pattern. Discuss contraception options with your GP or prescriber.
Could my period changes be from something other than Mounjaro?
Yes. Menstrual changes can be caused by stress, exercise changes, thyroid issues, polyps, fibroids, hormonal changes, or weight loss itself — not necessarily Mounjaro. Your GP can run tests (thyroid function, iron levels, ultrasound if needed) to identify the cause and ensure there's no underlying health issue.
Ready to take control of your weight loss journey?
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