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 Legs vs Wegovy: What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, and both can occasionally cause leg swelling or fluid retention as a side effect. This guide compares the two medications factually, covering their mechanisms, clinical trial data, and how your GP or prescriber will decide which is suitable for you. Individual results and side effects vary significantly between people.
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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 — And Why It Matters
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This means it activates two natural hormone pathways in your body — both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). This dual mechanism may help regulate appetite, reduce food cravings, and slow gastric emptying more effectively than a single-pathway medication.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. Clinical trials (the STEP series) showed average weight loss of around 14.9% body weight over 68 weeks at the maximum 2.4 mg dose. Mounjaro's SURMOUNT trials showed average weight loss of 15–20.9% depending on dose, suggesting slightly stronger results on average, though individual responses vary considerably. Both work alongside reduced-calorie diet and increased physical activity — neither works in isolation.
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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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Leg Swelling and Side Effects: Direct Comparison
Leg swelling (peripheral oedema) is not listed as a common or very common side effect of either medication in UK clinical data. However, fluid retention and swelling can occasionally occur with GLP-1 and GIP/GLP-1 medications. If you experience new or worsening leg swelling, swelling in your feet, or unexplained weight gain despite reduced eating, contact your GP immediately — this could indicate a more serious condition unrelated to the injection.
Both medications share common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain (especially early in treatment). Mounjaro side effects may be slightly more severe initially due to rapid dose escalation, but individual tolerance varies widely. Your prescriber will monitor you and can adjust doses if side effects become problematic.
Which Is Right for You? Prescriber DecisionYour GP or prescriber will assess which medication suits you based on your individual health profile, not on marketing or preference. Both require clinical assessment before prescription — neither is available without a prescription from a UK-licensed prescriber. Use our free GLP-1 weight loss calculator to explore potential weight loss ranges at different doses, and our BMI calculator to understand your current health category. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, cardiovascular disease). Your prescriber will also consider any other medications you take and your medical history.
- 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.
Does Mounjaro cause more leg swelling than Wegovy?
Leg swelling is not a common side effect of either medication according to UK trial data. Both can rarely cause fluid retention. If you experience new leg swelling on either injection, contact your GP — it may indicate a separate health issue needing investigation, not a typical medication side effect.
Which causes fewer side effects — Mounjaro or Wegovy?
Both have similar side effect profiles: nausea, diarrhoea, vomiting, stomach pain, and constipation are very common early on. Individual tolerance varies widely. Some people tolerate one better than the other, but this is unpredictable. Your prescriber may switch you if side effects are severe.
Does Mounjaro work faster than Wegovy?
Mounjaro's dose escalation is faster (4 weeks per dose step vs 1 week for Wegovy), so you may reach effective doses sooner. However, SURMOUNT trial data suggests Mounjaro may produce slightly greater average weight loss. Clinical outcomes vary significantly between individuals — weight loss depends on diet, activity, adherence, and individual metabolism.
Can I switch from Wegovy to Mounjaro if Wegovy isn't working?
Yes — prescribers can switch medications if one isn't producing adequate results or side effects are problematic. There's no official conversion chart, but approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg). Any switch must be managed by your prescriber, not self-initiated.
How do I get Mounjaro or Wegovy prescribed in the UK?
NHS access is through specialist tier 3 weight management services (refer via your GP, but waiting lists vary by region). Private access is through online consultations with UK-licensed prescribers followed by dispensing via a GPhC-registered pharmacy. Both require clinical assessment and confirmation that you meet eligibility criteria.
Ready to take control of your weight loss journey?
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