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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How To Change From Mounjaro To Wegovy
Switching between weight management injections requires careful medical oversight. If you're currently on tirzepatide (Mounjaro) and considering switching to semaglutide (Wegovy), this decision must be made in consultation with your prescriber. Both are prescription-only GLP-1 medications approved in the UK, but they work through different mechanisms and require individual assessment. This guide explains the practical and clinical differences to help you have an informed conversation with your healthcare provider.
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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.
Understanding The Mechanisms And Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways to regulate appetite and blood sugar. Clinical trials (SURMOUNT-1) showed average weight loss of approximately 15% at 5mg, 19.5% at 10mg, and 20.9% at 15mg over 72 weeks. Wegovy (semaglutide) is a single GLP-1 receptor agonist with a simpler mechanism. The STEP-1 trial showed average weight loss of approximately 14.9% at the maximum dose of 2.4mg over 68 weeks. Neither medication assures specific outcomes — individual results vary significantly, and both require a reduced-calorie diet and increased physical activity to be effective. Your prescriber considers your personal response, side effect tolerability, and health profile when advising on any switch. Use our GLP-1 weight loss calculator to explore projected outcomes at different dose levels for comparative reference.
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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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Dosing Equivalence And The Switch Process
There is no official established conversion between tirzepatide and semaglutide doses — approximate equivalences exist in clinical practice (5mg Mounjaro ≈ 0.5mg Wegovy; 10mg ≈ 1.7mg; 15mg ≈ 2.4mg) but these are not rigid. If your prescriber recommends a switch, the process typically involves a washout period of 3–7 days, then starting Wegovy at a lower dose (0.25mg) and titrating upward over 16+ weeks. This prevents compound side effects and allows your body to adjust to the new mechanism. Your prescriber will monitor your blood sugar control, appetite response, and any gastrointestinal symptoms during the transition. Never attempt to bridge doses or self-adjust — always follow your prescriber's specific 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.
Why would someone switch from Mounjaro to Wegovy?
Common reasons include persistent side effects on tirzepatide, insufficient weight loss response, cost considerations, or personal preference. Some people tolerate GLP-1-only medications better than dual-action medications. Your prescriber can assess whether a switch is clinically appropriate based on your individual response and health needs.
Can I take Mounjaro and Wegovy at the same time?
No. Taking both simultaneously would double the GLP-1 receptor stimulation and carry unacceptable safety risks. Any switch must involve stopping one medication before starting the other, with appropriate medical supervision and timing guidance from your prescriber.
How long does the switch typically take?
A full switch usually takes 3–4 weeks for the washout and initial titration on Wegovy. However, reaching an effective maintenance dose on Wegovy takes 16–20 weeks total. Your prescriber will establish a personalised timeline based on your tolerability and response.
Will I experience different side effects on Wegovy compared to Mounjaro?
Possibly. Both cause nausea, vomiting, diarrhoea, and constipation, but Mounjaro's dual mechanism may produce different tolerability profiles. Some people find the single GLP-1 action easier to tolerate; others prefer tirzepatide. Side effects typically improve as your body adjusts to the new medication.
Is Wegovy available on the NHS in the UK?
Wegovy access on the NHS is through specialist tier 3 weight management services following GP referral. However, availability and waiting times vary significantly by region. Private prescription access is available through UK-licensed online prescribers, typically faster than NHS specialist referral pathways.
Ready to take control of your weight loss journey?
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