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 Doesn't Mounjaro Work For Some People?
Not everyone loses the same amount of weight on Mounjaro, and some people see very limited results. This is completely normal — Mounjaro (tirzepatide) is effective for many people, but effectiveness varies significantly between individuals. Understanding why, and what this means under NHS guidance, can help you make informed decisions about whether this medication is right for you.
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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.
Individual Response & Clinical Trial Reality
Clinical trials show that Mounjaro helps most people lose weight, but the SURMOUNT-1 trial data reveals important variation. On average, adults taking 15 mg achieved approximately clinically studied weight loss — but this is an average across all participants. Some people lost significantly more, whilst others lost far less or experienced minimal change.
- Around 50% of trial participants achieved 15% or more weight loss
- Approximately 20% of participants achieved less than clinically studied weight loss
- Dose titration, lifestyle factors, and individual biology all influence outcomes
- Non-response does not mean the medication is "faulty" — it reflects natural variation in how bodies respond to GLP-1 and GIP signalling
Additionally, results plateau over time. Initial gradual weight management typically slows after 12–16 weeks, and many people reach a natural weight loss plateau where further loss becomes minimal despite continuing 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.
Reasons Why Some People See Limited Results
Several factors influence whether Mounjaro is effective for you personally: genetics affect how your body responds to GLP-1/GIP signalling; baseline metabolism determines how much deficit a given dose creates; lifestyle adherence is critical — Mounjaro works best alongside reduced calorie intake and increased activity; underlying medical conditions like PCOS, thyroid disorders, or certain medications may reduce effectiveness; and dose adequacy matters — some people may need 15 mg but only reach 10 mg due to side effects.
If you're on Mounjaro and experiencing minimal weight loss after 16+ weeks at an adequate dose, your prescriber may recommend stopping treatment, trying a different medication, or investigating underlying medical factors.
- 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.
Does non-response to Mounjaro mean I should never try weight loss medication?
No. Non-response to one medication doesn't predict non-response to another. Wegovy (semaglutide) works differently — it's a GLP-1 agonist only, not dual GIP/GLP-1 like Mounjaro. Some people respond better to single-mechanism medications. Your GP or prescriber can discuss alternatives based on your individual response.
How long should I give Mounjaro before deciding it's not working?
Most guidance suggests at least 16 weeks on an adequate dose (typically 10–15 mg) to assess effectiveness. You should see initial weight loss within the first 4–8 weeks. If you've reached the maximum tolerated dose and seen less than clinically studied weight loss after 16 weeks, discuss stopping with your prescriber — continuing may not be beneficial.
Can I use our GLP-1 weight loss calculator to estimate my results on Mounjaro?
Our calculator provides general estimates based on trial averages, but individual results vary widely. It's a useful reference point, but your actual outcome will depend on your personal factors, adherence, and dose tolerability. Use it as a starting reference, not a assures.
What should I do if Mounjaro isn't working after 4 months?
First, confirm you're on an adequate dose — at least 10 mg, ideally 15 mg if tolerated. Check that you're maintaining a calorie deficit alongside the medication. If both are true and weight loss remains negligible, contact your prescriber. They may recommend stopping, switching to Wegovy, investigating underlying medical causes, or considering alternative approaches.
Are some people medically unable to benefit from Mounjaro?
Yes. People with certain conditions may see limited benefit — severe gastroparesis, history of pancreatitis, or uncontrolled thyroid disease can reduce effectiveness. Some medications (like certain antipsychotics) work against weight loss. Your prescriber assesses this before prescribing. If you have multiple comorbidities, NHS eligibility and suitability should be thoroughly reviewed with a tier 3 specialist.
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.