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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Changing From Wegovy To Mounjaro
If you're considering a change from Wegovy to Mounjaro, it's important to understand how these prescription weight loss medications differ. Both are licensed for weight management in the UK, but they work through different mechanisms and may suit different individuals. This guide explains the differences between semaglutide (Wegovy) and tirzepatide (Mounjaro), compares clinical trial results, and outlines what to expect when switching. Your prescriber will assess whether a change is appropriate for your situation.
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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.
Mechanism Differences and Trial Data
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway to reduce appetite and support feeling fuller after smaller meals. The STEP-1 clinical trial showed an average weight loss of approximately 14.9% body weight at 68 weeks on the 2.4 mg maintenance dose.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways, which may provide additional appetite regulation and metabolic effects. The SURMOUNT-1 clinical trial showed average weight loss of approximately 20.9% at 72 weeks on the 15 mg dose, with 19.5% at 10 mg.
This dual mechanism is the key difference: tirzepatide activates both GIP and GLP-1 pathways, while semaglutide activates GLP-1 only. Individual responses vary significantly — some people achieve better results on one medication than the other. Your prescriber will consider your personal response, any side effects experienced, and your health goals when deciding if a switch is appropriate.
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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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Switching Considerations and Your Prescriber
Switching between weight loss medications requires guidance from your UK prescriber — whether through NHS specialist services or private online assessment. There is no official established conversion dose between Wegovy and Mounjaro, though approximate equivalences exist (for example, Mounjaro 10 mg is sometimes considered roughly equivalent to Wegovy 2.4 mg, and Mounjaro 15 mg may produce greater effects).
Common reasons for switching include: inadequate weight loss on current dose, persistent side effects, or wanting to try a dual-mechanism medication. Your prescriber will assess your current dose, how long you've been on it, any side effects, and your clinical history. A switch typically requires restarting at a lower titration dose rather than jumping to an equivalent higher dose, to allow your body to adjust. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses — this can help guide your discussion with your prescriber.
- 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 stronger than Wegovy?
Both are effective, but they work differently. Mounjaro (tirzepatide) has a dual mechanism activating two hormone pathways, while Wegovy (semaglutide) activates one. Clinical trials suggest Mounjaro may produce slightly greater average weight loss, but individual results vary significantly. Neither is universally "stronger" — suitability depends on your personal response and health circumstances.
Can I switch from Wegovy to Mounjaro on the NHS?
Possibly, through NHS specialist weight management services, though availability varies by region. Your GP or specialist can discuss switching if Wegovy hasn't produced adequate results or side effects are problematic. Private prescribing also offers switching options following a fresh clinical assessment.
What dose of Mounjaro should I start if switching from Wegovy?
You typically restart Mounjaro at a lower titration dose (2.5 mg) rather than jumping to an "equivalent" higher dose. There is no official conversion chart between medications. Your prescriber will advise based on your previous response and any side effects experienced on Wegovy. Dose escalation follows the standard titration schedule.
Will I experience different side effects on Mounjaro compared to Wegovy?
Both medications commonly cause nausea, diarrhoea, constipation, and stomach discomfort, particularly during dose escalation. Some people find side effects improve on Mounjaro if they've had persistent issues on Wegovy, while others experience similar or different side effects. Individual tolerance varies — your prescriber can advise based on your history.
How long should I try Mounjaro before deciding if it's working?
Most prescribers recommend at least 4 weeks on each dose before increasing, allowing time to assess effectiveness and tolerance. Significant weight loss typically becomes noticeable after 8–12 weeks on a stable dose. If Mounjaro isn't working adequately after 3–6 months at therapeutic doses, discuss other options with your prescriber.
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