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 Mounjaro To Wegovy
If you're considering switching from Mounjaro to Wegovy, understanding the key differences between these two prescription weight management injections is important. Both are licensed in the UK and work through different mechanisms to support weight loss, but they have distinct properties. Your GP or prescriber will assess whether a switch is appropriate for your individual circumstances. This guide explains how they compare and what you should know about making a transition between them.
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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 Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism may help regulate appetite more effectively through multiple signals in the body. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. In clinical trials, Mounjaro (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight at the maximum dose, whilst Wegovy (STEP-1 trial) showed approximately 14.9% average weight loss. However, individual results vary significantly. Both require a reduced-calorie diet and increased physical activity to achieve these outcomes. The choice between them often depends on how your body responds, your tolerability of side effects, and your prescriber's clinical judgment about which mechanism may suit you better.
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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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Key Practical Differences When Switching
Switching between these medications requires prescriber oversight — never stop one and start another without guidance. Dosing schedules differ: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg weekly, whilst Wegovy starts at 0.25 mg and escalates to 2.4 mg weekly. There is no official established conversion chart between them, though approximate equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy; 15 mg Mounjaro ≈ 2.4 mg Wegovy). Your prescriber may restart you at a lower dose on the new medication rather than attempting a direct dose conversion. Both are once-weekly injections, so your injection routine remains the same. Common side effects are similar (nausea, diarrhoea, constipation, stomach pain), though individual tolerability differs. Track any changes using our free progress tracker to monitor how you respond after switching.
- 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.
Why would I switch from Mounjaro to Wegovy?
Common reasons include side effects that are difficult to tolerate with Mounjaro, lack of expected weight loss response, or a prescriber's recommendation based on your individual health profile. Some people respond better to a single mechanism (Wegovy) than a dual mechanism (Mounjaro). Your GP or prescriber can discuss whether a switch is appropriate for you.
Can I switch directly from one dose of Mounjaro to an equivalent Wegovy dose?
There is no official established conversion between these medications. Your prescriber will decide whether to start Wegovy at a lower dose or at an equivalent tier based on your tolerability and response. Never attempt to convert doses yourself — always follow prescriber guidance.
Will I lose more weight on Wegovy than Mounjaro?
Clinical trial averages suggest Mounjaro may support slightly more weight loss overall, but individual results vary considerably. Some people respond better to Wegovy, others to Mounjaro. Weight loss depends on diet, activity, dose tolerability, and individual metabolism — not just the medication itself.
Do Mounjaro and Wegovy have the same side effects?
Both cause similar common side effects (nausea, diarrhoea, constipation, stomach pain), but individual tolerability differs significantly. Some people find one medication easier to tolerate than the other. Your prescriber monitors your response and adjusts accordingly.
How long should I wait between stopping Mounjaro and starting Wegovy?
Your prescriber will advise on timing. Generally, there may be minimal gap between stopping one and starting another, but this depends on your individual situation. Do not self-manage the switch — always follow your prescriber's guidance on timing and dosing.
Ready to take control of your weight loss journey?
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