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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Not Losing Weight on Mounjaro — What to Know
If you're not seeing the weight loss results you expected from Mounjaro, you're not alone. While clinical trials showed that most people lost weight on this weekly weight loss injection, individual responses vary significantly. This guide explains why weight loss may plateau or be slower than expected, what factors affect results, and what steps you can take through the NHS or with your prescriber to address the issue.
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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.
Understanding Weight Loss on Mounjaro — NHS Context
Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medication that works as a dual GIP and GLP-1 receptor agonist. In the SURMOUNT-1 clinical trial, participants on the 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks. However, this was achieved alongside a reduced-calorie diet and increased physical activity — not by the medication alone.
If you're not losing weight as quickly as expected, several factors may be at play: you may still be in the early titration phase (doses start at 2.5 mg and increase gradually every 4 weeks), your body may need more time to adjust, dietary intake may not reflect a true calorie deficit, or underlying health conditions may affect weight loss. Through NHS specialist tier 3 weight management services, your prescriber can review your progress and make adjustments.
- Weight loss typically takes 4–8 weeks to become noticeable
- Dose increases happen every 4 weeks minimum — rushing this reduces effectiveness
- Plateau periods are normal during long-term weight loss
- Medication works best with consistent lifestyle changes
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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.
What to Do If Mounjaro Isn't Working for You
If you've been on Mounjaro for at least 12 weeks at a stable dose without meaningful progress, contact your GP or the NHS specialist service that prescribed it. Your prescriber may: increase your dose to the next level (following the standard titration schedule), review your diet and activity levels to ensure a genuine calorie deficit exists, check for medical conditions affecting weight loss (such as thyroid problems or polycystic ovary syndrome), or assess whether the medication remains appropriate for you.
Calculate your personal BMI using our free BMI calculator to track your current status, and use our GLP-1 weight loss calculator to see projected outcomes at different doses. Keep a food diary if possible — it often reveals hidden calorie intake. Remember: sustainable weight loss averages 0.5–1 kg per week. Patience and consistency matter more than speed.
- 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.
How long does it take to see weight loss on Mounjaro?
Most people begin to notice weight loss within 4–8 weeks, though some take longer. The first 4 weeks are typically an adjustment period. If you haven't seen any change after 12 weeks at a stable dose, discuss this with your GP or prescriber — a dose increase or lifestyle review may help.
Why is my weight loss slower than expected on Mounjaro?
Common reasons include being in the early titration phase (doses increase gradually), not maintaining a calorie deficit despite appetite suppression, underlying medical conditions affecting metabolism, or simply individual variation. Weight loss is not linear — plateaus are normal. Your prescriber can help identify the cause.
Should I increase my Mounjaro dose if I'm not losing weight?
Never increase your dose without your prescriber's guidance. The standard schedule is to increase every 4 weeks minimum. Doses start at 2.5 mg and go up to a maximum of 15 mg. Your prescriber will assess whether increasing is safe and appropriate for you.
Can I get Mounjaro through the NHS if I'm not losing weight with Wegovy?
Yes. Both Mounjaro and Wegovy are available through NHS tier 3 specialist weight management services, though availability varies by region. NICE has approved both medications. If one isn't working, your NHS specialist service can assess whether switching to the other is appropriate.
Is Mounjaro right for me if I'm not seeing results?
Not necessarily. If you've been on a stable dose for 12+ weeks without progress, your prescriber may suggest stopping and trying a different approach, or they may identify a barrier (like insufficient calorie deficit or a medical condition) that can be addressed. Always discuss with your GP or prescriber before making changes.
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.