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 Affect Your Periods? A UK Comparison Guide
Yes, Mounjaro may affect your menstrual cycle, though this is not a direct effect of the medication itself. Instead, gradual weight management caused by tirzepatide (the active ingredient in Mounjaro) can sometimes disrupt periods in some women. Wegovy (semaglutide), another GLP-1 medication for weight management, can have similar effects. Both are prescription-only injections available in the UK following clinical assessment. Understanding how these medications work and their side effects helps you make an informed decision with your GP or prescriber. Use our free BMI calculator to check your eligibility and explore our GLP-1 weight loss calculator to understand potential outcomes.
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Calculate Your BMI Instantly
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 Mounjaro and Wegovy Work — Clinical Trial Context
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways in the body. This dual mechanism may contribute to faster weight loss than single-agent GLP-1 medications. In the SURMOUNT-1 clinical trial, participants on Mounjaro lost an average of 15–21% of their body weight over 72 weeks, depending on dose.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. In the STEP-1 clinical trial, participants lost an average of approximately 15% of body weight at 68 weeks on the maximum dose of 2.4 mg.
Because Mounjaro's dual mechanism may result in more gradual weight management on average, some women may experience greater disruption to their menstrual cycle compared with Wegovy. However, individual results vary significantly, and not all women experience menstrual changes. Any menstrual disruption is typically temporary and usually resolves once weight stabilises.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Menstrual Changes and Weight Loss — What the Evidence Shows
gradual weight management can temporarily disrupt menstrual cycles in some women because hormones like oestrogen are stored in fat cells. When fat loss happens quickly, hormone levels fluctuate, which may delay or skip periods or cause lighter or heavier bleeding. This effect is not specific to Mounjaro or Wegovy — it can occur with any gradual weight management method.
Both medications list menstrual irregularities as a possible side effect in clinical trials, though this is not the primary mechanism of action. If you experience concerning menstrual changes while taking either medication, inform your GP or prescriber promptly. They can assess whether the change is weight-related or requires further investigation. Track your progress with our free progress tracker to monitor both weight and symptoms over time.
- 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 cause periods to stop completely?
Temporary cessation is possible with gradual weight management, but complete or permanent loss of periods is uncommon. If your periods stop, inform your GP — this may indicate you've lost weight too quickly or there's an underlying issue unrelated to the medication. Your prescriber can adjust your dose or recommend a slower weight loss pace.
How quickly do menstrual changes happen after starting Mounjaro or Wegovy?
Menstrual disruption typically occurs after 4–12 weeks, as weight loss becomes noticeable. Some women see changes sooner, others later. The timing depends on how quickly you lose weight and your individual hormone sensitivity. Track your cycle and weight together to identify any patterns.
Will my periods return to normal after stopping Mounjaro or Wegovy?
Yes. Once you stop the medication and weight stabilises, menstrual cycles typically return to their previous pattern within 2–3 months. If periods don't resume within this timeframe, discuss it with your GP to rule out other causes.
Which is more likely to affect periods — Mounjaro or Wegovy?
Mounjaro may cause more noticeable menstrual changes because it's a dual-action medication and may support faster weight loss on average. However, individual responses vary widely — some women on Wegovy experience significant changes, whilst others on Mounjaro notice nothing. Your prescriber can discuss your personal risk factors.
Should I avoid Mounjaro or Wegovy if I'm concerned about menstrual disruption?
Not necessarily. Temporary menstrual changes are often mild and resolve quickly. If you have concerns, discuss them with your GP or prescriber before starting. They can monitor you during treatment and adjust your dose or approach if needed. The health benefits of weight loss may outweigh temporary disruption for some women.
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.