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 You Might Not Be Losing Weight on Mounjaro
If you're taking Mounjaro and not seeing the weight loss results you expected, you're not alone. Weight loss on any prescription weight loss injection — including Mounjaro (tirzepatide) — varies significantly between individuals. Some people lose weight steadily, whilst others experience slower progress or plateaus. This guide explains why this happens, how Mounjaro compares to Wegovy, and what conversations to have with your prescriber. Understanding these differences helps you make informed decisions about your weight management journey.
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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 — How the Medications Compare
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weekly weight loss injections licensed in the UK, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways — whilst Wegovy is a GLP-1 receptor agonist only. In clinical trials, both showed meaningful weight loss: Mounjaro at 15 mg dose showed approximately 20.9% average body weight loss (SURMOUNT trial), and Wegovy at 2.4 mg showed approximately 14.9% average weight loss (STEP trial). However, these are trial averages with a reduced-calorie diet and increased physical activity. Individual results vary considerably. Some people respond very well to one medication but not the other, which is why prescriber assessment and dose titration are crucial. Your GP or weight management specialist can help determine which is more suitable for your individual circumstances.
- Mounjaro: dual mechanism, maximum dose 15 mg weekly
- Wegovy: single GLP-1 mechanism, maximum dose 2.4 mg weekly
- Both require prescription and clinical assessment
- Both work best with lifestyle changes, not alone
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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 Weight Loss Plateaus Happen — And What to Do
Weight loss plateaus are common on any weight management medication, and there are several reasons why. First, your body adapts to medication over time — the appetite suppression effect may reduce as your body adjusts. Second, adherence matters — missing doses, not following the reduced-calorie diet, or inconsistent physical activity will slow progress. Third, dose optimisation is key — you may still be on a dose below your therapeutic level. Mounjaro dose titration takes up to 20 weeks to reach the maximum 15 mg dose, and some people need to stay on higher doses for maximum effect. Fourth, other health factors — stress, sleep, certain medications, or underlying health conditions — can affect weight loss. If you're not seeing progress after 12 weeks on your current dose, discuss this with your prescriber. They may recommend increasing your dose, extending your time on the current dose, or reviewing your diet and activity levels.
Using our free BMI calculator on this page can help track your progress objectively — sometimes progress appears as improved body composition rather than just scale weight. Our GLP-1 weight loss calculator can also help you understand realistic expectations based on medication type and dose. Keep a progress tracker to monitor changes in measurements, how clothes fit, and energy levels, not just weight. Weight loss is rarely linear, and small losses every few weeks are still progress.
- 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 less effective than Wegovy for weight loss?
Not necessarily. Both medications can be effective, but they work via different mechanisms. Clinical trial data shows Mounjaro (tirzepatide) produced slightly higher average weight loss percentages than Wegovy (semaglutide), but individual responses vary. Some people respond better to one than the other. Your prescriber will assess which is more suitable for you based on your health history and circumstances.
How long does it take to see weight loss on Mounjaro?
Most people begin to see weight loss within 2–4 weeks of starting Mounjaro, though this varies. Weight loss typically becomes more noticeable after 8–12 weeks as your body adjusts and your dose increases during the titration schedule. If you haven't seen progress by 12 weeks on your current dose, discuss this with your GP or weight management specialist.
Can I switch from Mounjaro to Wegovy if I'm not losing weight?
Switching between medications is possible, but it requires clinical assessment by your prescriber. They will consider your current response, any side effects, and your individual suitability. Don't stop or switch medications without consulting your GP or specialist — they can guide you safely and may adjust your current dose first.
What should I discuss with my prescriber if Mounjaro isn't working?
Be honest about your dose, adherence, diet, and activity level. Discuss whether you're at a therapeutic dose yet, if you need more time on your current dose, or if dose adjustment is needed. Also mention any health changes — stress, sleep problems, new medications — that might affect weight loss. Your prescriber may recommend lifestyle review, blood tests, or medication review.
Is a weekly weight loss injection the only option if Mounjaro doesn't work?
No. Your prescriber can explore several options: adjusting your Mounjaro dose higher, switching to Wegovy or Saxenda (liraglutide — a daily injection), reviewing your diet and exercise approach, or exploring other medical support through NHS tier 3 weight management services. Treatment should be personalised to your needs.
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.