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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How Does Mounjaro Work For Weight Loss?
Mounjaro (tirzepatide) is a weekly weight loss injection that works differently from other prescription weight loss medicines available in the UK. Unlike standard GLP-1 receptor agonist medications, Mounjaro uses a dual mechanism to help regulate appetite and support weight management. Understanding how Mounjaro works, and how it compares to alternatives like Wegovy, can help you discuss options with your GP or prescriber. Both are prescription-only medicines requiring clinical assessment, and individual results vary significantly. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses.
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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: How They Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—this dual action is what makes it distinct. It activates two natural hormone pathways in your body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This combination may help regulate appetite more effectively and support feeling fuller after smaller meals.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—it activates the single GLP-1 pathway. Both medications slow gastric emptying (how quickly food leaves your stomach), but Mounjaro's dual mechanism offers a different approach.
Clinical trial results: In the SURMOUNT-1 trial, Mounjaro at 15 mg helped participants lose an average of approximately 20.9% of body weight over 72 weeks. Wegovy's STEP-1 trial showed average weight loss of approximately 14.9% at 2.4 mg over 68 weeks. However, individual results vary widely—these are averages, not assures—and both medications work best alongside a reduced-calorie diet and increased physical activity.
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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.
Choosing Between Mounjaro and Wegovy in the UK
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections available in the UK through NHS specialist weight management services or private prescribing following clinical assessment. Eligibility criteria are similar: adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure.
The choice between them is not about which is "better"—it's about individual suitability. Your prescriber will assess your medical history, current medications, and health goals to recommend the most appropriate option. Some patients respond better to Mounjaro's dual mechanism, others to Wegovy's single pathway. Doses differ too: Mounjaro ranges from 2.5 mg to 15 mg weekly; Wegovy ranges from 0.25 mg to 2.4 mg weekly. Use our free BMI calculator to check your current status and discuss your weight management options with your GP.
- 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 Mounjaro more effective than Wegovy for weight loss?
Clinical trials show Mounjaro may support slightly higher average weight loss (approximately 15–21%) compared to Wegovy (approximately 15%), but individual results vary significantly. Both are effective; suitability depends on your personal health circumstances and how your body responds. Your prescriber will recommend the best option for you.
Why does Mounjaro have a dual mechanism when Wegovy only has one?
Mounjaro's dual GIP and GLP-1 action may help regulate appetite and energy through two hormone pathways simultaneously, potentially offering a different weight regulation approach. Wegovy's single GLP-1 pathway is well-established and effective. Neither is superior—they work differently, and individual response varies.
Can I switch between Mounjaro and Wegovy if one doesn't work for me?
Possibly, but this requires prescriber guidance. There is no official conversion chart between the medications. If one isn't working or causing side effects, your prescriber may suggest switching, but this must be managed carefully with proper dose adjustment and monitoring. Never switch without clinical guidance.
How long does it take to see weight loss results with Mounjaro?
Most people begin noticing appetite changes within 1–2 weeks; measurable weight loss typically appears after 3–4 weeks of consistent use alongside dietary and activity changes. However, timelines vary. Full results are usually assessed after 12–16 weeks of stable dosing. Patience and consistency are important.
Do I need a prescription to get Mounjaro in the UK, and how do I access it?
Yes, Mounjaro is prescription-only in the UK. Access is through NHS specialist tier 3 weight management services (via GP referral) or private prescribing following an online clinical assessment with a UK-licensed prescriber. Both routes require clinical assessment to confirm suitability.
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.