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
Can Mounjaro Not Work? Understanding Effectiveness
Yes, Mounjaro may not work effectively for everyone—and that's important to understand before starting. Weight loss medications like tirzepatide work differently depending on individual factors including metabolism, lifestyle, genetics, and how your body responds to the dual GIP and GLP-1 mechanism. Unlike assured treatments, Mounjaro delivers average weight loss results in clinical trials, but individual outcomes vary significantly. Some people lose substantial weight, while others see modest results or plateau. Your GP or prescriber can help assess whether Mounjaro is right for you based on your health profile and previous weight loss attempts. Our GLP-1 weight loss calculator can help you explore potential 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.
SURMOUNT vs STEP Trial Data: Mounjaro and Wegovy Compared
Clinical trial evidence shows different average results for Mounjaro (tirzepatide) and Wegovy (semaglutide), both UK prescription weight loss injections.
SURMOUNT-1 trial (Mounjaro): At 72 weeks on 15 mg, participants lost an average of 20.9% body weight alongside a reduced-calorie diet and increased physical activity. At the 5 mg starting dose, average loss was approximately 15%.
STEP-1 trial (Wegovy): At 68 weeks on 2.4 mg, participants lost an average of 14.9% body weight under the same lifestyle conditions.
The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist (activating two hormone pathways), whilst Wegovy is a GLP-1 agonist only. This dual action may explain why some people respond better to Mounjaro, but individual response varies. Some people respond well to Wegovy; others prefer Mounjaro based on personal tolerance and results.
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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 Might Not Work for You: Key Factors
Mounjaro effectiveness depends on several factors beyond the medication itself:
- Lifestyle adherence: Clinical trials combined Mounjaro with a reduced-calorie diet and exercise. Without these changes, weight loss is minimal.
- Genetics and metabolism: Some people's bodies are naturally more resistant to GLP-1 and GIP signalling, reducing effectiveness.
- Dose tolerance: Side effects (nausea, diarrhoea) may prevent reaching an effective dose for some users.
- Medical conditions: Certain health conditions, medications, or previous bariatric surgery can reduce response.
- Timing and consistency: Missing doses or inconsistent injection timing reduces effectiveness.
If Mounjaro isn't working after 3–4 months at an effective dose with good lifestyle adherence, your prescriber may suggest Wegovy or other options. A prescriber will assess individual suitability based on your health history.
- 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.
Why doesn't Mounjaro work for everyone?
Mounjaro's effectiveness depends on individual metabolism, lifestyle adherence, dose tolerance, and how your body responds to the dual GIP/GLP-1 mechanism. Clinical trials showed average weight loss, not assured outcomes. Genetics, medical conditions, and consistency with injections all affect results. Your prescriber can assess individual suitability.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) activates both GIP and GLP-1 hormone pathways (dual agonist). Wegovy (semaglutide) activates GLP-1 only. This dual action may help some people lose more weight on average in trials, but individual response varies. Some people respond better to Wegovy; it's a personal decision based on prescriber assessment.
Can I switch from Mounjaro to Wegovy if Mounjaro doesn't work?
Yes. If Mounjaro isn't effective after adequate time at an appropriate dose with good lifestyle adherence, your prescriber may suggest switching to Wegovy or another weight loss medication. The approximate Mounjaro 15 mg to Wegovy 2.4 mg conversion exists clinically, but any switch must be managed by your prescriber. Both require UK prescription and clinical assessment.
How long should I take Mounjaro before deciding it's not working?
Most prescribers assess effectiveness after 3–4 months at a stable, therapeutic dose. Weight loss should be at least 4% of initial body weight by 12 weeks to consider the medication effective. If you're not seeing results despite good diet, exercise, and dose compliance, discuss alternatives with your GP. Our progress tracker can help you log weight changes consistently.
Does my BMI affect whether Mounjaro will work?
BMI is one factor, but not the only predictor of Mounjaro effectiveness. Eligibility requires BMI 30+ or BMI 27–29.9 with weight-related health conditions (same for Wegovy). However, response to the medication depends more on individual metabolism, lifestyle factors, and dose tolerance than BMI alone. Use our free BMI calculator to check your starting point, then discuss realistic expectations with your prescriber.
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.