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
Free tracker included with every download this week
Does Mounjaro Stop Your Period?
Some women using Mounjaro (tirzepatide) report changes to their menstrual cycle, including delayed or stopped periods. Whilst not listed as a common side effect in the official product information, weight loss itself can affect hormone levels and menstruation. If your period has stopped or changed significantly since starting Mounjaro, it's important to understand what might be happening and when to contact your GP for guidance. This guide explains the connection between weight loss medication and menstrual changes.
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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.
How Weight Loss Affects Your Menstrual Cycle
gradual weight management can trigger menstrual irregularities, including delayed or stopped periods (amenorrhoea). This happens because oestrogen is stored in body fat — when fat stores decrease quickly, oestrogen levels may drop, affecting hormone signals that regulate your cycle. This is a known physiological response to significant weight loss, independent of any specific medication.
Mounjaro works by reducing appetite and supporting weight loss, so any weight loss it helps achieve could potentially affect your cycle in this way. However, the effect is from weight loss itself, not directly from tirzepatide. Factors affecting whether your period changes include: speed of weight loss (faster loss = higher risk), how much weight you've lost, your starting weight, and your individual hormone sensitivity.
- Gradual weight loss (0.5–1 kg per week) is less likely to cause menstrual disruption than rapid loss
- Very low-calorie diets increase the risk of stopped periods
- Stress, exercise intensity, and sleep also influence menstrual regularity alongside weight loss
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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.
Free Weight Loss Tools & Guides
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When to Contact Your GP About Menstrual Changes
If your period has stopped or changed significantly since starting Mounjaro, contact your GP — don't assume it's a normal side effect. Your GP needs to rule out other causes and assess whether your weight loss is too rapid or your overall health is affected.
Seek urgent medical advice if you experience: severe pelvic pain, heavy bleeding, signs of pregnancy despite contraception, or if amenorrhoea lasts longer than 3 months. Your GP can discuss whether to continue Mounjaro, adjust your dose, or modify your diet and exercise approach to slow weight loss. Women taking oral contraceptives should note that Mounjaro may reduce contraceptive absorption — use a barrier method or alternative contraception for 4 weeks after starting and 4 weeks after each dose increase.
- 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 a stopped period a known side effect of Mounjaro?
Stopped or delayed periods are not listed as a common side effect of tirzepatide in the official product information. However, they can occur as an indirect effect of gradual weight management. If your period changes after starting Mounjaro, the cause is likely the weight loss itself rather than the medication directly. Always inform your GP if your menstrual cycle changes.
How long does it take for periods to return after stopping Mounjaro?
If your period stopped due to gradual weight management, it typically returns once weight stabilises or loss slows down. This can take weeks to months depending on how much weight you've lost and how quickly. Every person is different — your GP can advise on your specific situation and whether investigation is needed.
Can I use Mounjaro if I'm trying to become pregnant?
No. Mounjaro is not recommended during pregnancy or if you're trying to conceive. If you're planning pregnancy, discuss this with your GP or prescriber before starting. You should also use contraception during treatment and for 4 weeks after your final dose due to reduced oral contraceptive absorption.
Why does gradual weight management stop periods?
Oestrogen is stored in body fat. When fat decreases rapidly, oestrogen levels drop significantly, disrupting the hormonal signals that trigger menstruation. This is a normal physiological response. Gradual weight loss (0.5–1 kg per week) is much less likely to cause menstrual disruption than faster weight loss.
Should I stop Mounjaro if my period stops?
Don't stop without speaking to your GP first. Your prescriber needs to assess whether the weight loss is too rapid, whether other causes are present, and what's best for your health. They may adjust your dose, slow your weight loss rate, or recommend other changes rather than stopping treatment entirely.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.