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
Why Isn't Mounjaro Working for Me — What You Need to Know
If you're on Mounjaro and not seeing the weight loss results you expected, you're not alone — individual responses to tirzepatide vary significantly. Weight loss injections work differently for different people, and sometimes a different medication may be more suitable. Understanding the differences between Mounjaro and alternatives like Wegovy, plus key factors that affect effectiveness, can help you have an informed conversation with your prescriber about next steps.
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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.
Mounjaro vs Wegovy — Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways. In the SURMOUNT-1 clinical trial, participants on 15mg Mounjaro lost an average of 20.9% of body weight over 72 weeks, alongside diet and exercise. However, not everyone responds the same way.
Wegovy (semaglutide) is a GLP-1-only receptor agonist — it targets a single hormone pathway. In the STEP-1 trial, participants on 2.4mg Wegovy lost an average of 14.9% of body weight over 68 weeks. Some people respond better to the single mechanism than the dual approach.
The key difference: Mounjaro's dual action may help more people feel fuller faster, but this also means Mounjaro causes nausea more frequently in some individuals. Wegovy's single mechanism may suit people who experience intolerable side effects on Mounjaro's KwikPen injections.
- Mounjaro: dual GIP + GLP-1, up to 15mg weekly
- Wegovy: GLP-1 only, up to 2.4mg weekly
- Both require prescription and clinical assessment
- Both work alongside reduced-calorie diet and physical activity
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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.
Why Mounjaro May Not Be Working — And What Happens Next
If Mounjaro isn't delivering results after adequate time at a therapeutic dose (typically 12–16 weeks on 10mg or higher), several factors may be involved: insufficient calorie deficit, poor medication absorption (especially if you're experiencing severe diarrhoea), dose titration plateau, or simply that your body responds better to a different medication class.
Your prescriber may suggest: escalating to a higher dose (up to 15mg), continuing longer to allow more time for response, or switching to an alternative like Wegovy or Saxenda. This is a clinical decision — your GP or prescriber will assess your individual suitability based on side effects, tolerance, and overall health. Use our GLP-1 weight loss calculator to understand expected outcomes at different doses, and track progress with our free progress tracker to identify genuine patterns.
The critical factor: weight loss medication is not a standalone solution. Mounjaro and Wegovy both require sustained calorie deficit and physical activity to work effectively. If you're not in a calorie deficit, neither medication will produce significant results — they support the process, they don't replace it. NHS guidance emphasises lifestyle change as the foundation.
Be honest with your prescriber about: adherence (are you taking every dose on schedule?), diet consistency, physical activity levels, and any side effects affecting compliance. If you're experiencing side effects severe enough to prevent you eating enough nutrition or engaging with your weight management programme, switching medications may be appropriate.
- 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 Mounjaro to Wegovy if Mounjaro isn't working?
Yes — switching is possible and managed by your prescriber. There is no official conversion chart, but approximate equivalence suggests Mounjaro 15mg is roughly equivalent to Wegovy 2.4mg. The switch requires clinical guidance on timing and does not happen immediately. Discuss options with your GP or the prescriber issuing your prescription.
How long should I give Mounjaro before deciding it's not working?
NHS guidance suggests at least 12 to 16 weeks at a therapeutic dose (10mg or higher) before assessing effectiveness. Some people respond slower than others. Weight loss is not linear — you may lose steadily for weeks, then plateau naturally. Use our free progress tracker to spot real patterns versus normal fluctuation.
Is Wegovy more effective than Mounjaro?
Trial data shows Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%), but individual results vary significantly. Some people respond much better to Wegovy's single mechanism, especially if they experience side effects on Mounjaro. Effectiveness is personal — what works best depends on your body and tolerance.
Could it be my diet and exercise, not the medication?
Yes, absolutely. Weight loss injections work only with a calorie deficit. If you're not in a calorie deficit — from diet, exercise, or both — neither Mounjaro nor Wegovy will produce meaningful results. Be honest with your prescriber about your daily intake and activity. A dietitian referral can help.
What should I do if Mounjaro and Wegovy both don't work?
Other UK-licensed options include Saxenda (liraglutide, a daily injection) or bariatric surgery through NHS tier 4 services if obesity and health complications are severe. However, exhausting all doses, ensuring genuine adherence, and confirming adequate calorie deficit and exercise are essential first. Discuss this with your GP.
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.