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 changing from Wegovy (semaglutide) to Mounjaro (tirzepatide), it's important to understand how the two medications differ and what the switch involves. Both are once-weekly weight loss injections licensed in the UK, but they work through different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. This guide explains the key differences, clinical trial results, and how your prescriber can assess whether switching is appropriate for you. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses of either medication.
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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 Wegovy And Mounjaro Differ — Mechanism And Trial Data
Mechanism: Wegovy activates the GLP-1 hormone pathway, reducing appetite and supporting fullness. Mounjaro activates both the GIP and GLP-1 pathways — a dual-action approach that may provide additional appetite regulation benefits. This fundamental difference means individual responses can vary between the two medications.
Clinical trial results: Wegovy (STEP-1 trial) showed average weight loss of approximately 14.9% body weight over 68 weeks at the maximum dose of 2.4 mg. Mounjaro (SURMOUNT-1 trial) showed average weight loss of approximately 15–21% body weight over 72 weeks depending on the final dose used (5 mg, 10 mg, or 15 mg).
- Wegovy maximum dose: 2.4 mg once weekly
- Mounjaro maximum dose: 15 mg once weekly (dose range is wider)
- Both require a reduced-calorie diet and increased physical activity to achieve results
- Individual results vary significantly between people
These trial averages demonstrate that Mounjaro has a wider dose range, but neither medication assures weight loss or is suitable for everyone.
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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.
When To Switch And What Your Prescriber Will Consider
Switching from Wegovy to Mounjaro is a prescriber decision based on your individual health, response to current treatment, side effects, and eligibility. You may be considered for a switch if: you haven't achieved desired weight loss on Wegovy despite being on the maximum dose for adequate time; you're experiencing side effects that limit your ability to continue Wegovy; or your prescriber believes the dual GIP+GLP-1 mechanism of Mounjaro may be more suitable for you.
Important: There is no automatic equivalent dose between the two medications. If you switch, your prescriber will likely restart you on Mounjaro's starting dose (2.5 mg) and titrate upwards over weeks, even if you were on a higher Wegovy dose. This allows your body to adjust to the new medication's mechanism. Always discuss timing, dose titration, and any gap between injections with your prescriber — never stop one medication and start another without medical guidance.
- 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?
Clinical trials suggest Mounjaro may support slightly higher average weight loss (15–21% vs 14.9%), but individual results vary considerably. Effectiveness depends on your personal response, adherence, diet, and activity level. Your prescriber can assess which medication may be more suitable for you based on your health profile.
Can I switch from Wegovy to Mounjaro on the NHS?
NHS access to both medications is through specialist tier 3 weight management services. Switching between medications is a prescriber decision made within NHS specialist care or private prescribing pathways. Regional availability varies significantly — discuss with your GP or specialist about whether switching is an option in your area.
What dose of Mounjaro should I start on if I switch from Wegovy?
Your prescriber will typically restart you on Mounjaro's starting dose of 2.5 mg, regardless of your Wegovy dose. This allows safe titration with the new dual-mechanism medication. Dose escalation typically occurs every 4 weeks. Never adjust doses yourself — always follow your prescriber's guidance.
Will switching from Wegovy to Mounjaro reduce my side effects?
Switching may improve some side effects but could introduce different ones. Both medications commonly cause nausea, diarrhoea, constipation, and stomach discomfort, especially during dose titration. Some people find side effects improve over time; others may find them reduced with a different medication. Discuss expected side effects with your prescriber.
How long should I stay on Wegovy before considering a switch to Mounjaro?
You should generally be on an adequate dose of Wegovy for at least 12 weeks (3 months) before discussing a switch, to allow time for your body to adjust and for results to become apparent. If you're experiencing intolerable side effects sooner, discuss this with your prescriber — they may recommend dose adjustment or switching earlier. ```
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