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 — What You Need to Know
If you're currently taking Wegovy (semaglutide) and considering a switch to Mounjaro (tirzepatide), this guide covers what you need to know about changing between these two GLP-1 weight loss medications in the UK. Both are prescription-only injectables, but they work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 only. Any change between medications must be managed by your prescriber following a clinical assessment. This article explains the NHS context, eligibility criteria, and practical considerations for switching.
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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 Wegovy and Mounjaro Differ in the UK
Wegovy (semaglutide) and Mounjaro (tirzepatide) are both prescription-only weight loss injections licensed in the UK, but they have key differences. Wegovy is a GLP-1 receptor agonist — it activates one hormone pathway to reduce appetite and slow gastric emptying. Mounjaro activates two pathways: both GIP and GLP-1, which may provide additional metabolic benefits. In the STEP-1 clinical trial, Wegovy at 2.4 mg produced average weight loss of approximately 14.9% body weight over 68 weeks. In the SURMOUNT-1 trial, Mounjaro at 15 mg produced average weight loss of approximately 20.9% body weight over 72 weeks. However, individual results vary significantly. The KwikPen format is identical for both — once-weekly injections. Your prescriber may recommend a switch if your current medication is not producing the desired results, or if you experience side effects that suggest a different medication might be better tolerated.
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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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NHS Prescribing and Switching Between Medications
In the UK, both Wegovy and Mounjaro are prescribed through NHS specialist tier 3 weight management services or privately through online prescribers. If you're accessing treatment via the NHS, any switch between medications requires a new assessment and approval from the specialist service. This process can take several weeks depending on your local integrated care board (ICB) and service availability. Privately, prescribers can facilitate a switch more quickly following a consultation. There is no official NHS conversion chart between Mounjaro and Wegovy — approximate equivalence exists (Wegovy 2.4 mg ≈ Mounjaro 10–15 mg), but your prescriber will assess whether to restart at a lower dose and titrate upward, or begin at a maintenance dose. Your GP can refer you back to NHS specialist services if you wish to switch via the NHS route.
- 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.
Can I switch from Wegovy to Mounjaro on the NHS?
Yes, but you'll need to be assessed by an NHS specialist tier 3 weight management service. Your GP can refer you, or if you're already receiving Wegovy through NHS services, request a medication review with your specialist team. Switching via NHS services may involve a waiting period.
What's the approximate dose equivalent between Wegovy and Mounjaro?
Wegovy 2.4 mg is approximately equivalent to Mounjaro 10–15 mg, though no official conversion chart exists. Your prescriber will decide whether to restart Mounjaro at a lower dose (e.g. 2.5 mg) and titrate upward, or begin at a higher maintenance dose. Individual factors guide this decision.
Will I experience different side effects with Mounjaro?
Possibly. Both medications cause nausea, diarrhoea, and constipation commonly, but the dual mechanism of Mounjaro may affect you differently than Wegovy. Some people tolerate one better than the other. Your prescriber will monitor you closely after switching.
How long does it take to see results after switching to Mounjaro?
Weight loss typically becomes noticeable within 4–8 weeks at a therapeutic dose. However, Mounjaro requires dose titration — you'll start at 2.5 mg and increase weekly until reaching your target dose, which delays full effect. Individual timelines vary.
What if I want to switch privately rather than via the NHS?
You can access private prescribing through a UK-licensed online prescriber following a clinical consultation. Private prescribers can facilitate switches more quickly than NHS services, though costs apply. Always ensure your chosen provider is GPhC-registered.
Ready to take control of your weight loss journey?
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