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 Has Mounjaro Stopped Working For Me?
If you've been taking Mounjaro and notice its effects have plateaued or diminished, you're not alone — weight loss medication tolerance is a recognised clinical phenomenon. This guide explains why this happens, how Mounjaro compares to Wegovy, and what your prescriber might recommend next. Both are prescription-only GLP-1 or dual receptor agonist medications licensed in the UK for weight management, but they work slightly differently and suit different individuals.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Mounjaro vs Wegovy: Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously, which is its key difference from Wegovy. Wegovy (semaglutide) is a single GLP-1 agonist. In the SURMOUNT-1 clinical trial, Mounjaro at maximum dose (15 mg) achieved approximately 20.9% average body weight loss over 72 weeks. Wegovy, tested in the STEP-1 trial, achieved approximately 14.9% average weight loss at its maximum dose (2.4 mg) over 68 weeks.
- Mounjaro: Dual mechanism may provide stronger appetite suppression initially, but tolerance can develop over time
- Wegovy: Single GLP-1 pathway; some prescribers use it as a switch option if Mounjaro efficacy declines
- Individual variation: Results vary significantly — what works powerfully for one person may work differently for another
The reason Mounjaro "stops working" is typically tachyphylaxis (tolerance development), where your body gradually adapts to the medication's effects, reducing its appetite-suppressing impact.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
What To Do If Mounjaro Efficacy Declines
If you're experiencing reduced effectiveness, speak to your prescriber before making any changes. They may recommend: increasing your dose if you haven't reached the 15 mg maximum; adjusting your diet to ensure you're maintaining a calorie deficit (tolerance to medication doesn't mean the medication stops working — it means you must be more disciplined with nutrition); or switching to Wegovy if Mounjaro has truly plateaued. Some prescribers also suggest a temporary break from the medication, though this should only be done under guidance. Your GP or weight management specialist will assess which option suits your individual health and progress using tools like our free GLP-1 weight loss calculator to project outcomes.
- 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 it normal for Mounjaro to stop working after a few months?
Yes, tolerance can develop with any GLP-1 or dual receptor agonist medication. Your body may adapt to the medication's effects, reducing its appetite-suppressing impact. This isn't medication failure — it's a recognised clinical phenomenon called tachyphylaxis. Your prescriber can adjust your approach, including dose escalation, dietary discipline, or exploring alternatives like Wegovy.
Should I switch from Mounjaro to Wegovy if tolerance develops?
Possibly, but only on your prescriber's recommendation. Wegovy uses a single GLP-1 mechanism rather than Mounjaro's dual GIP+GLP-1 pathway. Some individuals respond better to one or the other. Your prescriber will assess your individual suitability based on your health profile, previous response, and current weight management goals. Use our GLP-1 weight loss calculator to explore potential outcomes with different approaches.
What's the difference between Mounjaro and Wegovy dosing?
Mounjaro ranges from 2.5 mg to 15 mg once weekly. Wegovy ranges from 0.25 mg to 2.4 mg once weekly. The approximate equivalence is Mounjaro 10 mg ≈ Wegovy 1.7 mg, though these are not strict conversions. Any switch between medications must be managed by your prescriber — there is no official conversion chart, and individual tolerance differs significantly.
Can I increase my Mounjaro dose if it's stopped working?
If you haven't reached the 15 mg maximum dose, your prescriber may recommend dose escalation. However, if you're already on 15 mg, a dose increase isn't an option. In this case, your prescriber may suggest diet and activity optimisation, a medication break, or a switch to Wegovy or another licensed weight management medication.
Does reduced Mounjaro efficacy mean the medication is harmful?
No. Tolerance development is not a sign of harm — it's a physiological adaptation where your body becomes less responsive to the medication's effects. You may still benefit from continuing the medication at your current dose whilst focusing more intensively on dietary changes and physical activity, which remain the foundation of sustainable weight loss alongside any medication.
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.