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 Affect Your Periods? Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are weight loss injections used in the UK, and some women report changes to their menstrual cycle when starting these medications. This guide explains what we know about how each medication may affect periods, the mechanisms behind these changes, and what to expect. If you're considering either treatment, understanding potential effects on your cycle is an important part of making an informed decision with your prescriber.
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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 Mounjaro and Wegovy May Affect Menstrual Cycles
Neither Mounjaro nor Wegovy is specifically reported to directly alter hormone production, but gradual weight management itself can cause menstrual changes. Weight loss of more than 10% body weight or very rapid loss (over 1.5 kg per week) can temporarily disrupt periods by reducing oestrogen levels, which may lead to irregular cycles, lighter periods, or temporary amenorrhoea (absence of periods).
Both medications work through appetite suppression and slowed gastric emptying, leading to weight loss when combined with dietary changes. In clinical trials, women reported changes to their menstrual cycle, though these are not listed as common side effects in the formal product information. The underlying cause is typically the weight loss itself, not the medication directly.
- Mounjaro (tirzepatide) — dual GIP and GLP-1 receptor agonist, maximum dose 15 mg weekly
- Wegovy (semaglutide) — single GLP-1 receptor agonist, maximum dose 2.4 mg weekly
- Both are once-weekly injections approved for weight management in UK adults
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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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Individual Variation and Prescriber Guidance
Every woman responds differently to weight loss medications. Some experience no change to their periods, whilst others notice irregular cycles during the initial weight loss phase. Periods often normalise as weight stabilises. If you're taking oral contraceptives, important interactions exist—Mounjaro may reduce absorption of oral medications including the contraceptive pill, so your prescriber may recommend additional barrier contraception for 4 weeks after starting and after each dose increase.
Your GP or prescriber can assess your individual health history and discuss potential effects before starting treatment. Use our free GLP-1 weight loss calculator to explore projected weight loss timelines, which helps contextualise the pace of change your body may experience. This information supports conversations with your healthcare team about managing expectations around menstrual changes.
- 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.
Can Mounjaro or Wegovy directly stop your periods?
No. Neither medication directly affects hormone production. Period changes are typically caused by gradual weight management reducing oestrogen levels, not the medication itself. This effect is usually temporary and often resolves as weight stabilises.
Which causes more menstrual disruption—Mounjaro or Wegovy?
Clinical evidence doesn't show one causing significantly more menstrual changes than the other. Both can result in period changes due to weight loss. Individual responses vary widely. Your prescriber can discuss your personal risk factors.
Is it safe to take weight loss injections if you're using the contraceptive pill?
Both medications may reduce absorption of oral contraceptives. The NHS advises adding a barrier method (condoms) or switching to a non-oral contraceptive for 4 weeks after starting and after each dose increase. Always inform your prescriber of all medications.
Will my periods return to normal after stopping Mounjaro or Wegovy?
In most cases, yes. Once weight stabilises and the rate of weight loss slows, menstrual cycles typically return to their previous pattern. If periods remain irregular months after stopping, discuss this with your GP.
Are there other side effects I should know about besides period changes?
Yes. Common side effects include nausea, diarrhoea, constipation, and stomach pain. Period changes are not officially listed as common side effects in the medication information, though some women report them. Your prescriber will discuss all potential effects before treatment begins.
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