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 Am I Not Losing Weight on Mounjaro?
Weight loss on tirzepatide (Mounjaro) isn't assured, and results vary significantly between individuals. If you're not seeing the progress you expected, it could be related to dose, time on treatment, lifestyle factors, or individual metabolism. Understanding how Mounjaro works compared to other options like Wegovy, and knowing when to discuss concerns with your prescriber, can help you make informed decisions about your weight management journey.
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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.
Mounjaro vs Wegovy: Trial Data and Mechanism Differences
Clinical trial data shows both medications can help with weight loss, but results differ. In the SURMOUNT-1 trial, Mounjaro at 15mg achieved average weight loss of approximately 20.9% of body weight over 72 weeks, whilst Wegovy (semaglutide) in the STEP-1 trial achieved approximately 14.9% average weight loss at the 2.4mg dose over 68 weeks. 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 see better results with Mounjaro, but it's not universal — individual response depends on genetics, metabolism, diet adherence, and physical activity. Neither medication assures specific weight loss, and clinical trial results represent averages. Your prescriber considers your individual suitability when choosing between them.
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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 You May Not Be Losing Weight on Mounjaro
Several factors can affect weight loss on Mounjaro: you may still be early in your treatment — meaningful changes often take 8–12 weeks; your dose may not yet be optimised — the Mounjaro KwikPen dose titration schedule increases weekly doses gradually, and some people need higher doses; lifestyle factors matter — the medication works alongside a reduced-calorie diet and increased physical activity, not instead of them; and individual metabolism varies significantly.
Some people simply respond better to different medications. If you've been on a stable, therapeutic dose for 12+ weeks with consistent diet and exercise, and weight loss has stalled, discuss this with your GP or prescriber. They may adjust your dose, explore underlying factors like thyroid function or medication interactions, or consider whether an alternative like Wegovy might suit you better.
- 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 see initial changes within 4–8 weeks, but meaningful weight loss typically emerges at weeks 8–12. You should be on a therapeutic dose (usually 5mg or higher) for at least 8–12 weeks before assuming the medication isn't working. Individual timelines vary significantly, and patience is important.
Is Wegovy better than Mounjaro for weight loss?
SURMOUNT trial data showed Mounjaro achieved slightly higher average weight loss (~21%) than Wegovy (~15%), but individual results vary considerably. Neither is universally "better" — your prescriber assesses which suits your health profile, dose tolerance, and medical history. Trial averages don't predict your personal outcome.
What dose of Mounjaro is most effective for weight loss?
The maximum dose is 15mg once weekly, but many people see good results at 10mg or 12.5mg. Dose titration is gradual — you increase every 4 weeks. Higher doses aren't always more effective for every individual. Your prescriber will optimise based on your progress and tolerability of side effects.
Can I switch from Mounjaro to Wegovy if I'm not losing weight?
Yes, it's possible to switch, but only under prescriber guidance. There's no direct dose conversion between them. If you've been on an optimised Mounjaro dose for 12+ weeks without progress, discuss switching with your GP or prescriber — they'll assess whether Wegovy might be more suitable and manage the transition safely.
Does the Mounjaro side effect nausea affect weight loss?
Nausea is very common early on and typically reduces over 4–8 weeks. It can limit how much you eat, which may initially aid weight loss, but severe nausea that prevents adequate nutrition is counterproductive. Report persistent severe nausea to your prescriber — dose adjustment or anti-nausea support may help.
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.