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 Weight Gain: How It Compares to Wegovy
If you're considering a weekly weight loss injection in the UK, you may wonder whether Mounjaro (tirzepatide) or Wegovy (semaglutide) is more effective at preventing weight gain or supporting weight loss. Both are prescription-only medications licensed for weight management in UK adults, but they work differently. This guide explains the key differences, clinical trial data, and how prescribers choose between them. Both require clinical assessment and work best alongside diet and physical activity 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.
Clinical Trial Data: SURMOUNT vs STEP
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone pathways in the body. In the SURMOUNT-1 clinical trial, adults receiving the maximum 15mg dose lost an average of approximately 20.9% of their body weight over 72 weeks. Those on 10mg lost around 19.5%, and those on 5mg lost roughly 15% of body weight.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—activating a single hormone pathway. In the STEP-1 trial, adults on the maximum 2.4mg dose lost an average of approximately 14.9% of body weight over 68 weeks. Both trials included participants following a reduced-calorie diet and increased physical activity.
These are clinical trial averages; individual results vary significantly. Weight loss outcomes depend on your starting point, adherence, lifestyle changes, and how your body responds to the medication.
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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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How They Work Differently
The key difference is mechanism of action. Mounjaro targets two receptor pathways (GIP and GLP-1), which may explain why trial data showed numerically higher average weight loss. Wegovy targets one pathway (GLP-1 only). Both medications slow gastric emptying (how quickly food leaves your stomach), reduce appetite, and support feeling fuller after smaller meals.
For UK adults, both are available through NHS specialist tier 3/4 weight management services or private prescription following an online clinical assessment. Your prescriber will assess which is more suitable based on your individual health profile, BMI, any existing health conditions, and current medications. Use our GLP-1 weight loss calculator above to explore potential outcomes at different doses for either medication.
Neither medication assures weight loss—both work alongside, not instead of, lifestyle changes. A weight loss injection is most effective when combined with a balanced diet, reduced calories, and regular physical activity. Both Mounjaro and Wegovy require a prescription from a UK-licensed prescriber. Access varies: NHS availability is through specialist services (waiting lists apply in many regions); private access is available online following clinical assessment.
Your GP can provide a referral to NHS specialist services, or you can explore private options. Try our free BMI calculator and weight converter above to understand your current health baseline before speaking with a prescriber.
- 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 Mounjaro better than Wegovy for preventing weight gain?
Clinical trials suggest Mounjaro may result in numerically higher average weight loss (up to 20.9% vs 14.9%), but individual results vary significantly. "Better" depends on your personal health profile, existing medications, and how your body responds. Both are effective; your prescriber will recommend the most suitable option after clinical assessment.
Why does Mounjaro have higher trial weight loss numbers?
Mounjaro works on two hormone pathways (GIP + GLP-1), while Wegovy works on one (GLP-1 only). The dual mechanism may contribute to higher average weight loss in trials. However, individual responses differ; some people achieve excellent results on either medication.
Can I switch from Wegovy to Mounjaro on the NHS?
Potentially, yes—but this requires your NHS specialist prescriber's decision. There is no official NHS protocol for switching between medications. If you're on Wegovy and considering Mounjaro, discuss this with your specialist weight management service. Approximate dose equivalencies exist but aren't standardised.
How long does it take to see weight loss results on either injection?
Most people notice changes within 4–8 weeks, though individual timelines vary. Clinical trials measured results over 68–72 weeks. Both medications require at least 4 weeks at each dose before increasing. Consistent use alongside diet and activity changes gives the best results.
What's the difference between a weekly weight loss injection and daily medication?
Both Mounjaro and Wegovy are once-weekly injections, making them convenient compared to daily tablets. Liraglutide (Saxenda) is a daily injection also licensed in the UK. Once-weekly injections typically improve adherence and are often preferred by patients for ease of use.
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.