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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Can You Switch From Mounjaro To Wegovy In The UK?
Yes, some people do switch between Mounjaro and Wegovy in the UK, but any medication change must be managed by your prescriber. Both are prescription-only weight management injections licensed in the UK, but they work slightly differently and have different dosing schedules. This guide explains what switching involves, whether the NHS covers it, and what you need to know before considering a change.
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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 Differ — And Why People Switch
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways — while Wegovy (semaglutide) is a single GLP-1 receptor agonist. In clinical trials, Mounjaro achieved slightly higher average weight loss (around 20.9% at the maximum 15 mg dose) compared to Wegovy (around 14.9% at 2.4 mg). However, individual results vary significantly, and some people respond better to one than the other.
- Mounjaro: Once-weekly injection, doses range from 2.5 mg to 15 mg, quicker dose escalation
- Wegovy: Once-weekly injection, doses range from 0.25 mg to 2.4 mg, slower dose escalation over 16 weeks
- Side effects: Both cause similar gastrointestinal effects (nausea, diarrhoea), though some people tolerate one better than the other
People typically consider switching if they experience persistent side effects, reach a plateau in weight loss, or have cost concerns — especially when comparing NHS and private access options.
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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
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Switching Via The NHS — Availability And Eligibility
If you are accessing Mounjaro or Wegovy through an NHS tier 3 specialist weight management service, any switch between medications must be requested through your specialist team. NICE has approved both tirzepatide and semaglutide for NHS use, but availability varies by region (Integrated Care Board). Some areas may have restrictions on which medication is available first, or may require you to continue your current medication for a set period before considering alternatives.
Switching on the NHS typically involves a clinical review to assess whether the change is appropriate, your current progress, and your tolerance. There is no official established conversion dose between Mounjaro and Wegovy — your prescriber will restart you on the lowest dose of the new medication and escalate gradually. Expect a 4–8 week assessment period before any change is made.
- 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.
Has anyone successfully switched from Mounjaro to Wegovy?
Yes, some people do switch between these medications in the UK. Real-world experience varies — some report better side effect tolerance with Wegovy, while others prefer Mounjaro's stronger effect. Any switch must be managed by your prescriber, not self-initiated. Your GP or specialist team can discuss whether a change is appropriate for your situation.
What happens to my dose when I switch medications?
There is no direct dose conversion between Mounjaro and Wegovy because they work slightly differently (dual vs single mechanism). Your prescriber will restart you on the lowest dose of the new medication (Wegovy 0.25 mg or Mounjaro 2.5 mg) and titrate upwards gradually over weeks. This prevents side effects and allows your body to adjust.
Can I switch from Mounjaro to Wegovy on the NHS?
Possibly, but it depends on your regional NHS service. NICE has approved both medications, but local prescribing policies vary. Request a review with your tier 3 specialist team. They will assess whether switching is clinically appropriate, whether you've reached a treatment plateau, and your individual tolerance. This typically requires a 4–8 week review.
Why would someone switch from Mounjaro to Wegovy?
Common reasons include persistent nausea or gastrointestinal side effects, cost concerns (private Wegovy may be cheaper than Mounjaro at higher doses), reaching a weight loss plateau, or access issues. Some people tolerate one medication better than the other — switching allows your prescriber to find the right fit for your body.
Can I switch back to Mounjaro if Wegovy doesn't work?
Yes, switching between medications is possible — it's not a one-way decision. If Wegovy doesn't deliver the results you hoped for after adequate dosing and time (typically 16+ weeks), your prescriber can reassess and consider returning to Mounjaro or exploring other options. Always discuss this with your clinical team before making any changes.
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