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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Mounjaro From Doctors — How It Compares to Wegovy
When you discuss weight management with your GP or a specialist prescriber, you may hear about two main prescription weight loss injections available in the UK: Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are licensed for weight management, but they work differently and suit different people. This guide explains the key differences doctors consider when prescribing, helping you understand what your GP or prescriber might recommend and why. Use our GLP-1 weight loss calculator to explore projected outcomes with either medication.
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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 Mounjaro and Wegovy Work Differently
The most important difference between these two medications is how they work at the cellular level. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two different hormone pathways in your body. Wegovy is a GLP-1 receptor agonist only, activating a single pathway. This dual mechanism in Mounjaro may provide a different weight loss profile and hunger response compared to Wegovy's single GLP-1 action.
In clinical trials, Mounjaro (SURMOUNT-1 trial) showed average weight loss of approximately 15–21% of body weight at the maximum 15 mg dose. Wegovy (STEP-1 trial) showed average weight loss of approximately 15% of body weight at its maximum 2.4 mg dose. Results vary significantly between individuals — some people respond better to one medication than the other. Doctors may choose based on individual factors including previous medication response, side effect tolerance, and injection schedule preference.
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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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What Your GP Considers When Prescribing
Your prescriber will assess your eligibility, health history, and personal circumstances. Both Mounjaro and Wegovy require a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, hypertension, or cardiovascular disease. Ethnicity-adjusted thresholds apply for South Asian, Chinese, Black African, and Black Caribbean patients — your GP will discuss this if relevant.
Doctors also consider side effect profiles. Both medications commonly cause nausea, diarrhoea, and constipation, especially at higher doses. Some patients tolerate one better than the other. Mounjaro requires fewer injections per month (once weekly) compared to Wegovy's once-weekly schedule — both are equal here. Cost, availability through NHS specialist services or private prescription, and your preference all influence the final recommendation. Use our BMI calculator to check your starting point and understand your eligibility category.
- 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 I switch from Wegovy to Mounjaro if I'm not seeing results?
Yes, switching between medications is possible but must be managed by your prescriber. There is no standard conversion chart between the two — your GP will assess whether a switch is appropriate based on your response, side effects, and overall health. This is typically done under specialist supervision, not in routine GP practice.
Which medication does the NHS prefer — Mounjaro or Wegovy?
The NHS has approved both medications for specialist tier 3/4 weight management services. However, NHS availability and prescribing preference vary significantly by region and individual ICB guidance. Private prescribing through online GPhC-registered pharmacies offers access to both without NHS waiting lists.
Are the side effects the same for both Mounjaro and Wegovy?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain — these are the most frequent side effects. Individual tolerance varies. Some patients report fewer or milder side effects with one medication compared to the other, though clinical data shows similar overall side effect profiles.
Which one gives faster weight loss — Mounjaro or Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (up to 21% vs 15%), but individual results vary enormously. Faster weight loss is not always better — sustainable loss at 0.5–1 kg per week is the NHS recommendation. Your prescriber will prioritise steady, safe progress over gradual weight management.
Do I need to take both medications together?
No — you take either Mounjaro or Wegovy, not both. They are alternative medications, not complementary. Your prescriber will recommend one based on your individual assessment. Both must be used alongside a reduced-calorie diet and increased physical activity for best results.
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.