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 Not Working — Should You Switch to Wegovy?
If Mounjaro isn't delivering the weight loss results you expected, you're not alone — individual responses to GLP-1 and GIP/GLP-1 medications vary significantly. Before considering a switch to Wegovy or another option, it's important to understand whether your dose is optimised, how long you've been taking it, and what realistic expectations are. This guide compares Mounjaro and Wegovy directly to help you understand the differences and what to discuss with your prescriber. Our free GLP-1 weight loss calculator can help you explore projected outcomes at different doses.
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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.
Mounjaro vs Wegovy — Mechanism and Trial Results
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work through different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways to regulate appetite and support weight loss. Wegovy is a GLP-1 receptor agonist only, activating a single pathway. Clinical trial data shows different average outcomes: in the SURMOUNT trials, Mounjaro at 15 mg achieved approximately 20.9% average body weight loss, while in the STEP trials, Wegovy at 2.4 mg achieved approximately 14.9% average weight loss. However, these are clinical trial averages — individual results vary widely. Some people respond excellently to Mounjaro, others achieve better results with Wegovy. Response depends on personal metabolism, genetics, lifestyle factors, and adherence to diet and exercise changes alongside 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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Dose Titration and Timeline — Is Mounjaro Really Not Working?
Before concluding that Mounjaro isn't working, consider whether your dose is fully optimised. Mounjaro requires careful dose titration — starting at 2.5 mg, then increasing by 2.5 mg increments every 4 weeks up to the maximum 15 mg. If you're on a lower dose (5–7.5 mg), significant weight loss may not yet be evident. Allow at least 12–16 weeks at your current dose before expecting full results. Some people lose weight steadily throughout, others plateau and then lose again. Weight loss is rarely linear. Use our free progress tracker to log weekly measurements — sometimes non-scale changes (how clothes fit, energy levels) appear before scale movement. If you've been on a stable, therapeutic dose (10 mg or higher) for 12+ weeks with no progress despite calorie deficit and exercise, discuss with your prescriber about increasing further or exploring alternatives like Wegovy.
- 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.
How long does Mounjaro take to work?
Most people see initial appetite suppression within 1–2 weeks, but meaningful weight loss typically takes 4–8 weeks. Maximum effect is usually seen at 12–16 weeks on a stable dose. Weight loss is gradual and individual — don't expect rapid results. If you're on a low starting dose, allow 4 weeks between increases before assessing effectiveness.
Can I switch from Mounjaro to Wegovy if Mounjaro isn't working?
Yes, switching is possible and your prescriber can manage this. The approximate Mounjaro 10 mg equivalent is Wegovy 1.7 mg, though there's no official conversion chart. Any switch requires prescriber guidance. Some people do respond better to one over the other — this is why an individual trial period matters.
Is Mounjaro 15 mg always better than lower doses?
Not necessarily. Clinical trials show higher average weight loss at 15 mg, but some people achieve good results at 5–10 mg. Side effects (nausea, constipation) often increase at higher doses. Your prescriber will help find the dose that balances efficacy with tolerability for your individual situation.
What if I've gained weight on Mounjaro?
Weight gain on Mounjaro despite being on a therapeutic dose is unusual but can happen due to appetite return, reduced adherence to diet, or underlying conditions. Discuss with your prescriber — they may increase your dose, review your diet and exercise, or explore other causes. Mounjaro dose titration requires time and patience.
Does Wegovy work better than Mounjaro for everyone?
No — both work well for some people and less well for others. Wegovy may suit you if you respond to GLP-1 alone, or if you want a longer established medication (Wegovy trials date back further). Your prescriber will help assess which is more appropriate based on your health, previous response, and eligibility.
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.