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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Not Losing Weight On Mounjaro — How It Compares To Wegovy
If you've been taking Mounjaro without seeing weight loss, you may wonder whether switching to an alternative like Wegovy could help. Both are prescription weight loss injections licensed in the UK, but they work differently and produce different results in clinical trials. Understanding the differences — and why individual responses vary so much — can help you and your prescriber decide what's right for you. This guide compares the two medications factually, using real trial data and NHS context.
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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.
Clinical Trial Results: SURMOUNT vs STEP
Mounjaro (tirzepatide) and Wegovy (semaglutide) achieved different weight loss outcomes in their respective clinical trials, though both showed meaningful results when combined with diet and lifestyle changes.
SURMOUNT-1 trial (Mounjaro): Participants taking the maximum dose (15 mg once weekly) lost an average of approximately 20.9% of body weight over 72 weeks. The 10 mg dose achieved around 19.5% loss, and the starting 5 mg dose produced roughly 15% loss. These results were achieved alongside a reduced-calorie diet and increased physical activity.
STEP-1 trial (Wegovy): Participants on the maximum dose (2.4 mg once weekly) lost an average of approximately 14.9% of body weight over 68 weeks, also with diet and exercise support. Wegovy's mechanism is simpler — it's a single GLP-1 receptor agonist, whereas Mounjaro works as a dual GIP and GLP-1 agonist, which may explain the slightly higher average weight loss seen in trials.
However, individual results vary considerably. Some people lose significantly more, others less. Not everyone responds equally to either medication, even at maximum doses.
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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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Why You Might Not Be Losing Weight, And What To Consider Next
Several reasons explain why you might not be seeing weight loss on Mounjaro, even at higher doses:
- Dose timing: You may still be in the titration phase — Mounjaro requires a minimum of 4 weeks on each dose before escalating. Full effect at higher doses can take 8–12 weeks.
- Dietary adherence: The medication only works when combined with a reduced-calorie diet. Without a consistent calorie deficit, weight loss won't occur.
- Individual variation: Some people are less responsive to GLP-1 and GIP pathways — this is biological, not a failure on your part.
- Underlying conditions: Certain medications, thyroid issues, or other health factors can affect weight loss response.
- Switching options: A prescriber may suggest trying a weekly weight loss injection like Wegovy, or exploring other tier 3 weight management support. Any switch must be discussed with your prescriber.
Use our GLP-1 weight loss calculator to compare projected weight loss across different doses of both medications, and try our BMI calculator to track your current health metrics while you work with your GP or prescriber on next steps.
- 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 Wegovy more effective than Mounjaro if I haven't lost weight on Mounjaro?
Trial data shows Mounjaro achieved higher average weight loss (20.9% vs 14.9%), but individual responses vary widely. Some people respond better to one than the other. Effectiveness depends on your biology, adherence to diet changes, and dose. Your prescriber can assess whether switching makes sense for you specifically.
What's the key difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two appetite-regulating pathways. Wegovy is a GLP-1-only agonist, activating one pathway. The dual mechanism may contribute to Mounjaro's slightly higher trial weight loss, but both are effective prescription weight loss injections available in the UK through NHS specialist services or private prescription.
How long does Mounjaro take to work for weight loss?
Most people notice appetite changes within 1–2 weeks of starting, but significant weight loss typically emerges over 8–12 weeks as doses are gradually increased. The dose titration process takes 20+ weeks to reach maximum. If you're in the early phases, continue following your reduced-calorie diet while your dose escalates. If you've been on a stable dose for 8+ weeks with no change, discuss options with your prescriber.
Can I switch from Mounjaro to Wegovy if Mounjaro isn't working?
Yes — your prescriber can switch you to Wegovy or explore other weight management options. There's no official conversion formula between the medications, but approximate equivalences exist (Mounjaro 15 mg ≈ Wegovy 2.4 mg). Any change must be managed by your prescriber. Switching typically involves a washout period and starting the new medication's dose titration protocol.
Should I increase my Mounjaro dose if I'm not losing weight yet?
Only if you're not yet at the maximum dose (15 mg) and haven't spent at least 4 weeks on your current dose. Titration matters — rushing doses or skipping the gradual escalation won't improve results. If you've reached 15 mg and spent 8+ weeks on maximum dose without meaningful weight loss, discuss next steps with your prescriber. They may explore diet adherence, underlying health factors, or alternative treatments.
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.