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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Switching From Wegovy To Mounjaro — What You Need To Know
If you're considering switching from Wegovy to Mounjaro, you're looking at two different prescription weight loss injections licensed in the UK. Wegovy (semaglutide) is a GLP-1 receptor agonist, whilst Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist — meaning it works on two hormone pathways instead of one. Both are once-weekly injections, but they differ in mechanism, doses, and clinical trial results. Switching requires prescriber guidance and individual clinical assessment. This guide compares the two medications factually to help you understand the differences.
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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 Wegovy and Mounjaro Work — Key Differences
Wegovy (semaglutide) activates the GLP-1 hormone pathway only. GLP-1 signals to your brain that you are full, slows stomach emptying, and helps regulate blood sugar. The STEP-1 clinical trial showed participants on Wegovy 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways — this dual mechanism is its distinguishing feature. The SURMOUNT-1 clinical trial showed participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks under similar conditions. However, individual results vary significantly regardless of which medication is used.
- Wegovy: single hormone pathway (GLP-1)
- Mounjaro: dual hormone pathways (GIP + GLP-1)
- Both UK prescription-only medications
- Both require clinical assessment before starting
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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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Dose Ranges and Side Effects — What Differs
Wegovy doses range from 0.25 mg to a maximum of 2.4 mg weekly. Mounjaro doses range from 2.5 mg to a maximum of 15 mg weekly — so the dosing scale is different. Both medications commonly cause nausea, diarrhoea, constipation, vomiting, and stomach pain, particularly during dose escalation. Hair loss has been reported with both medications. The severity and duration of side effects vary between individuals. Switching from one medication to another involves stopping one and starting the other — your prescriber will guide the timing and starting dose for the new medication.
Your prescriber will consider your individual health, any previous side effects you experienced with Wegovy, and whether Mounjaro is clinically suitable before recommending a switch. Use our free GLP-1 weight loss calculator to explore potential weight loss projections at different Mounjaro doses.
- 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.
Can I switch directly from Wegovy to Mounjaro without a break?
Not necessarily. The timing and approach to switching depends on your individual circumstances. Your prescriber may suggest a washout period between stopping Wegovy and starting Mounjaro, or they may recommend starting Mounjaro at a low dose shortly after stopping Wegovy. Never switch medications without prescriber guidance. Timing matters for your safety and effectiveness.
Is Mounjaro more effective than Wegovy?
Clinical trials suggest Mounjaro may support slightly higher average weight loss (approximately 20.9% vs 14.9%), but individual results vary significantly. Some people respond better to one medication than another. Effectiveness depends on your personal health, metabolism, and adherence to diet and exercise changes. Your prescriber can discuss which might be more suitable for you.
Are side effects worse with Mounjaro than Wegovy?
Both medications cause similar side effects — nausea, diarrhoea, constipation, and stomach pain are common. Some people report fewer side effects when switching medications; others report similar or different side effects. Your experience is individual. If you had significant side effects with Wegovy, discuss this with your prescriber before switching to Mounjaro.
How much does Mounjaro cost in the UK privately?
Private Mounjaro costs vary by dose and provider, typically ranging from £129 to £320 per month. Lower doses (2.5 mg–5 mg) are generally cheaper than higher doses (15 mg). Prices vary between private prescribing services, so compare before committing. NHS access is through specialist weight management services and availability varies by region.
Will my GP refer me to Mounjaro if I've already tried Wegovy?
Possibly. NHS referral to specialist tier 3 weight management services depends on your circumstances, the reason you're switching (ineffectiveness, side effects, or other factors), and your local NHS ICB's policies. Private prescribers can assess you directly without NHS referral. Ask your GP about NHS specialist services or consult a private prescriber for assessment.
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