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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Why Does Weight Loss Slow Down on Mounjaro?
Weight loss on Mounjaro may slow after the first few months — this is normal and expected. Like any weight loss medication, your body adapts over time, and results naturally plateau as you reach a new weight set point. Understanding what slowing weight loss means, comparing it to Wegovy outcomes, and knowing when to talk to your prescriber can help you stay motivated and make informed decisions about your treatment. This guide explores the mechanisms behind slowing weight loss and how Mounjaro stacks up against other GLP-1 injections.
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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 Data: Mounjaro vs Wegovy Weight Loss Patterns
In the SURMOUNT-1 clinical trial, Mounjaro (tirzepatide) showed an average weight loss of approximately 20.9% of body weight over 72 weeks at the maximum 15 mg dose — alongside a reduced-calorie diet and increased physical activity. By comparison, the STEP-1 trial for Wegovy (semaglutide) demonstrated approximately 14.9% average weight loss over 68 weeks at the 2.4 mg maintenance dose under similar conditions.
The key difference lies in mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously. Wegovy is a GLP-1-only agonist, working through a single pathway. This dual action may explain why Mounjaro showed numerically higher average weight loss in trials — though individual results vary significantly, and some people respond better to one medication than the other.
Both medications show a pattern of steeper initial weight loss (weeks 1–16) followed by a more gradual plateau phase. This is not failure — it reflects the body's natural adaptation as weight stabilises at a new set point. Most weight loss occurs during dose escalation; once on maintenance dose, progress naturally slows.
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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.
Individual Suitability: Why Your Prescriber Decides
Whether slowing weight loss on Mounjaro means you need to switch to Wegovy (or vice versa) is a clinical decision between you and your prescriber — not based on trial averages, but on your individual response, medical history, and health goals.
Some people plateau on Mounjaro and lose more on Wegovy; others see the reverse. Factors that influence which injection suits you include existing medical conditions, concurrent medications, side effect tolerance, and how your body responds to GIP and GLP-1 stimulation. A prescription weight loss injection UK prescriber will assess whether continuing Mounjaro, increasing the dose, or switching medications is appropriate for you.
Using our free GLP-1 weight loss calculator and progress tracker can help you monitor your actual results and discuss trends with your healthcare provider — concrete data is more useful than comparing yourself to trial averages.
- 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 slowing weight loss on Mounjaro normal?
Yes. After initial rapid loss during dose escalation, weight loss naturally slows as your body adapts and reaches a new weight set point. Clinical trials show this pattern — steep loss weeks 1–16, then gradual plateau. This is expected physiological adaptation, not medication failure. Discuss any concerns with your prescriber.
How does Mounjaro weight loss compare to Wegovy?
SURMOUNT-1 showed Mounjaro averaging ~20.9% loss; STEP-1 showed Wegovy averaging ~14.9%. However, individual results vary widely. Some people lose more on Wegovy, others on Mounjaro. Your prescriber will assess which is suitable based on your personal response, not trial averages.
Can I switch from Mounjaro to Wegovy if weight loss slows?
Only on medical advice. Slowing is normal and doesn't automatically indicate you need to switch. Your GP or prescriber will assess whether continuing Mounjaro, adjusting your dose, or switching to a different weekly weight loss injection UK option is appropriate for your individual circumstances.
What is the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Wegovy (semaglutide) is GLP-1-only, activating one pathway. The dual action in Mounjaro may contribute to higher average weight loss in trials, but individual suitability varies.
How do I know if my Mounjaro weight loss has genuinely slowed?
Track your weight consistently using our free progress tracker — weekly or fortnightly measurements show patterns better than daily weigh-ins. Slowing is normal after 3–4 months. If you have concerns, discuss actual data with your prescriber rather than comparing to trial 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.