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 Has Mounjaro Stopped Working — Comparing With Wegovy
If you're on Mounjaro (tirzepatide) and weight loss has plateaued, you're not alone — weight loss plateaus are common on any medication. Understanding whether your body has adapted to your current dose, whether a dose increase is needed, or whether switching to Wegovy (semaglutide) might help are important conversations to have with your prescriber. This guide explains the differences between these two UK-licensed weight loss injections and what happens when one stops producing results.
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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.
Understanding Weight Loss Plateaus and Mounjaro vs Wegovy
Weight loss plateaus happen because your body adapts to medication over time — a process called tolerance. On Mounjaro, a plateau does not mean the medication has "stopped working" entirely; it usually means your body has reached equilibrium at your current dose or lifestyle.
The key difference between Mounjaro (tirzepatide) and Wegovy (semaglutide) is their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways, whilst Wegovy is a GLP-1 receptor agonist only. Clinical trial data shows Mounjaro achieved average weight loss of approximately 20.9% at the maximum 15 mg dose (SURMOUNT-1 trial), whilst Wegovy achieved approximately 14.9% at 2.4 mg (STEP-1 trial). However, individual results vary significantly. Your prescriber may recommend: continuing your current Mounjaro dose with stricter diet and exercise adherence, increasing to the next Mounjaro dose (up to 15 mg maximum), or exploring whether Wegovy might suit you better, though this requires clinical assessment and a new prescription.
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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.
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When to Speak to Your Prescriber About Mounjaro
If you've been on Mounjaro for at least 4–6 weeks at your current dose with no further weight loss, it's reasonable to discuss this with your prescriber. Do not increase your dose yourself — dose escalation must be managed by your healthcare professional and typically happens over weeks 4 or more on each dose. Keep a progress tracker of your weight, dietary intake, and exercise to share with your prescriber — this helps them understand whether a plateau is medication-related or lifestyle-related. Switching medications is a significant decision and requires clinical assessment; your prescriber will consider your health history, any side effects you've experienced, and your individual response before recommending Wegovy as an alternative.
- 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 has my weight loss stopped on Mounjaro?
Weight loss plateaus are normal and happen because your body adapts to medication. This doesn't mean Mounjaro has stopped working — it usually means you've reached equilibrium at your current dose. A plateau can be managed by increasing your Mounjaro dose (up to 15 mg maximum, under prescriber guidance), tightening dietary adherence, or increasing physical activity. Keep your prescriber informed.
Is Mounjaro or Wegovy more effective for weight loss?
Clinical trials show Mounjaro achieved approximately 20.9% average weight loss at maximum dose, whilst Wegovy achieved approximately 14.9%. However, individual results vary significantly. Some people respond better to one medication than the other. Suitability depends on your personal health profile, which your prescriber will assess during a clinical review.
Can I switch from Mounjaro to Wegovy?
Yes, switching between medications is possible but requires a new clinical assessment and prescription. Your prescriber will review your history on Mounjaro, any side effects, and whether Wegovy may be more suitable. There is no standard conversion chart — your prescriber will determine an appropriate starting dose for Wegovy if a switch is recommended.
How long should I wait before concluding Mounjaro isn't working?
Most guidance suggests giving any weight loss medication at least 4–6 weeks at a stable dose before evaluating effectiveness. Weight loss is rarely linear — plateaus can last weeks, then progress may resume with continued adherence. If you've been on your current Mounjaro dose for 8+ weeks with no progress and optimised diet and exercise, discuss options with your prescriber.
What's the difference between Mounjaro and Wegovy mechanism?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist — it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only — it activates one pathway. This dual action in Mounjaro may provide stronger appetite suppression for some people, but individual responses vary. Your prescriber will assess which mechanism suits your individual health needs.
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