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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What Mounjaro Does — How It Compares to Wegovy
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that works differently from Wegovy. Both are once-weekly injections approved for weight management, but Mounjaro uses a dual mechanism targeting two hormone pathways (GIP and GLP-1), whereas Wegovy targets GLP-1 only. Understanding how each medication works can help you and your GP or prescriber discuss which might be suitable for your individual circumstances. This guide explains the key differences in mechanism, clinical trial results, and practical considerations for UK patients.
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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.
How Mounjaro and Wegovy Work — The Key Difference
Mounjaro (tirzepatide) activates both GIP and GLP-1 receptor pathways. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are natural hormones that regulate appetite, food intake, and blood sugar. By activating both pathways simultaneously, Mounjaro may help reduce hunger, support feeling fuller after smaller meals, and slow how quickly food leaves the stomach.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates the GLP-1 pathway. GLP-1 alone also reduces appetite and supports satiety, but through a single mechanism rather than a dual approach.
- Mounjaro: Dual GIP + GLP-1 activation
- Wegovy: GLP-1 activation only
- Both: Once-weekly injection, prescription-only, require clinical assessment
In clinical trials, the dual mechanism of Mounjaro was associated with slightly higher average weight loss compared to semaglutide, though individual results vary significantly and depend on dose, lifestyle changes, and personal factors. Your prescriber will assess which approach may be more suitable for you.
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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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Clinical Trial Results and Individual Suitability
The SURMOUNT-1 clinical trial showed that participants taking Mounjaro at the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. The STEP-1 trial for Wegovy showed average weight loss of approximately 14.9% at the maximum 2.4 mg dose over 68 weeks under similar conditions.
However, these are trial averages — individual outcomes vary. Some people respond very well to one medication; others may respond better to the alternative. Both Mounjaro and Wegovy require a prescription following a clinical assessment from a UK-licensed prescriber. Use our free GLP-1 weight loss calculator to explore projected weight loss at different doses, and discuss with your GP or private prescriber which medication may be right for your health profile and goals.
- 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 better than Wegovy for weight loss?
Clinical trials suggest Mounjaro may support slightly higher average weight loss due to its dual mechanism, but individual results vary significantly. Both are effective; suitability depends on your personal health profile, medical history, and prescriber assessment. Neither is universally "better" — the right choice is individual.
Why does Mounjaro have a dual mechanism?
Mounjaro activates both GIP and GLP-1 pathways, which work together to regulate appetite, satiety, and blood sugar. The dual approach may provide additional benefit compared to GLP-1 activation alone, though this varies between individuals.
How long before Mounjaro starts working?
Many people notice reduced appetite within the first few days to one week. Weight loss typically becomes measurable within 2–4 weeks, though this varies. Mounjaro requires titration over several weeks at increasing doses before reaching maintenance levels.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. There is no official conversion chart, though approximate dose equivalences exist. Any switch requires clinical guidance to ensure safe transition and appropriate dosing.
Do I need a GP referral to get Mounjaro or Wegovy in the UK?
On the NHS, you typically need a referral to a specialist tier 3 weight management service. Privately, you can access both through online consultation with a UK-licensed prescriber without an NHS referral, though availability and costs vary by provider.
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.