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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Mounjaro Guidance: How Does It Compare to Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK for adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. The key difference lies in their mechanisms: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 alone. Both require clinical assessment before prescription, and individual results vary. This guidance explains how they differ and what might influence which could suit your circumstances. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your current status.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual receptor agonist, meaning it activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. This dual action may help regulate appetite more broadly and support fat utilisation, in addition to slowing gastric emptying.
Wegovy (semaglutide) is a GLP-1-only receptor agonist, activating one hormone pathway to reduce appetite and promote feeling fuller after smaller meals.
In clinical trials, both medications showed meaningful weight loss results:
- Mounjaro: SURMOUNT trials showed average weight loss of approximately 15–21% body weight (depending on dose)
- Wegovy: STEP trials showed average weight loss of approximately 15% body weight
However, clinical trial averages do not predict individual outcomes. Prescriber assessment will consider your medical history, current health conditions, and suitability for each medication.
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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.
Which Medication Might Suit You?
Both Mounjaro and Wegovy are prescribed based on individual assessment—there is no universal "better" option. Your prescriber will consider factors including your medical history, other medications, weight-related health conditions, and any previous weight loss treatment experience.
Mounjaro (tirzepatide): Dual mechanism; once-weekly 2.5–15 mg titratable doses; may be suitable if single GLP-1 activation hasn't worked previously.
Wegovy (semaglutide): Single GLP-1 mechanism; once-weekly 0.25–2.4 mg titratable doses; longer UK clinical track record; may be first-line choice.
Both require a prescription following a clinical assessment through either NHS specialist services or private online prescribing. Track your progress with our free progress tracker once treatment begins.
- 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.
What is the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (two hormone pathways), whilst Wegovy is a GLP-1-only agonist (one pathway). The dual mechanism may offer different effects on appetite and metabolism, but both support weight management when combined with lifestyle changes.
Which injection is more effective for weight loss?
Clinical trials show comparable results: Mounjaro averaged 15–clinically studied weight loss (SURMOUNT), and Wegovy averaged approximately 15% (STEP). Individual results vary significantly. Effectiveness depends on your personal health, adherence, and lifestyle. Your prescriber will recommend based on your circumstances.
Can I switch from Wegovy to Mounjaro if Wegovy isn't working?
Switching between medications is a decision for your prescriber. There is no official conversion chart—any switch requires medical guidance and reassessment. Approximate equivalences exist (e.g., Wegovy 1.7 mg ≈ Mounjaro 10 mg), but this is a clinical decision, not automatic.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both are available through NHS specialist tier 3 weight management services, accessed via GP referral. Availability and waiting times vary significantly by region. Check with your local ICB (Integrated Care Board) about services in your area and current waiting lists.
Are there side effects I should know about with either injection?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain, especially during dose titration. Side effects typically reduce over time. Serious side effects are rare but include pancreatitis symptoms and gallbladder issues. Always discuss concerns with your prescriber or GP immediately.
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.