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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15mg Mounjaro Not Working — When to Consider Alternatives
If you've been on the maximum 15mg Mounjaro dose and haven't seen the weight loss results you expected, it can be frustrating. Weight loss plateaus are common, and some people respond better to different medications. This guide compares Mounjaro with Wegovy — another prescription weight loss injection available in the UK — to help you understand your options and what to discuss with your prescriber.
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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.
Comparing Mounjaro and Wegovy: Mechanism and Trial Data
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both once-weekly GLP-1 based injections licensed for weight management in the UK, but they work slightly differently. Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways — while Wegovy is a GLP-1 agonist only. In clinical trials, Mounjaro at 15mg showed average weight loss of approximately 20.9% of body weight, whilst Wegovy at 2.4mg (maximum dose) showed average weight loss of approximately 14.9% over similar timeframes. However, individual results vary significantly — some people respond better to one mechanism than another. Clinical trial data comes from the SURMOUNT-1 trial (Mounjaro) and STEP-1 trial (Wegovy), both conducted alongside reduced-calorie diet and increased physical activity. Your response to either medication depends on individual factors including your body's sensitivity to these hormone pathways, adherence to lifestyle changes, and underlying health conditions.
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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.
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Why Weight Loss May Plateau and What to Discuss With Your Prescriber
A plateau after several months on maximum-dose Mounjaro can happen for several reasons: your body may have adapted to the medication, you may not be in sufficient calorie deficit despite the appetite suppression, or the dual GIP+GLP-1 mechanism may not be optimal for your individual physiology. Some people find switching to a single GLP-1 mechanism (like Wegovy) helps restart progress, whilst others need to reassess diet and activity levels. This is not a failure on your part — medication response is highly individual. Your prescriber can review your progress, discuss whether continuing Mounjaro or trying a different UK-licensed option like Wegovy might be suitable, and ensure any change is managed safely. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and track your progress with our progress tracker to identify true plateaus versus normal fluctuations.
- 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 might Mounjaro work for some people but not others?
Weight loss medication response depends on individual factors including genetics, hormonal sensitivity, adherence to diet and exercise, underlying health conditions, and how your body responds to GIP and GLP-1 receptor activation. The dual mechanism of Mounjaro works excellently for many people but not universally. Your prescriber can assess whether your plateau is due to medication response or other factors like insufficient calorie deficit.
Is Wegovy definitely better than Mounjaro if Mounjaro hasn't worked?
Not necessarily. Clinical trials show Wegovy and Mounjaro achieve similar or comparable average weight loss, though the mechanisms differ. Some people respond better to a single GLP-1 mechanism, others to the dual mechanism. There's no way to predict individually — it depends on your body's physiology. Your prescriber will assess suitability based on your health profile and previous response.
What should I do if 15mg Mounjaro isn't delivering results?
Schedule an appointment with your prescriber to review your progress. Discuss whether you're achieving a consistent calorie deficit through diet and activity, check for any underlying causes of weight loss resistance (like thyroid issues), and explore whether switching to Wegovy or continuing with lifestyle optimisation might help. Never stop or change medication without guidance.
How long should I wait before deciding Mounjaro "isn't working"?
Most weight loss is seen in the first 3–6 months. If you've been on 15mg for at least 3–4 months with no progress despite consistent lifestyle effort, it's reasonable to discuss alternatives with your prescriber. Some people see continued slow loss beyond this, so context matters. Your prescriber will help determine if a change is warranted.
Can I switch directly from 15mg Mounjaro to Wegovy?
Switching between medications must be managed by your prescriber — there is no automatic conversion between tirzepatide and semaglutide doses. Your prescriber will assess the timing, how to discontinue Mounjaro safely, and at what Wegovy dose to start. Never self-switch between prescription medications. The approximate equivalence is 15mg Mounjaro to 2.4mg Wegovy, but clinical decisions are individual.
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.