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- 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 in the UK
Some UK adults prescribed Mounjaro (tirzepatide) consider switching to Wegovy (semaglutide), either due to side effects, cost, availability, or clinical reasons. Both are prescription-only GLP-1 receptor agonists approved for weight management, but they work differently and have different dose ranges. This guide explains the NHS prescribing context, eligibility criteria, and what switching between these medications involves. Always discuss any medication change with your GP or prescriber—switching is a clinical decision that requires professional assessment.
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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.
NHS Prescribing, Eligibility, and Dose Considerations
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are licensed in the UK for weight management in adults with a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Access through the NHS is via tier 3 specialist weight management services following GP referral—availability and waiting times vary significantly by region. Privately, both are available through GPhC-registered online pharmacies following a clinical assessment by a UK-licensed prescriber.
Dose conversion between these medications is not straightforward. Mounjaro uses a dual GIP and GLP-1 mechanism with doses ranging from 2.5 mg to 15 mg weekly, while Wegovy is a GLP-1-only medication with doses from 0.25 mg to 2.4 mg weekly. Approximate clinical equivalence suggests 5 mg Mounjaro ≈ 0.5 mg Wegovy, 10 mg ≈ 1.7 mg, and 15 mg ≈ 2.4 mg, but any switch must be managed by your prescriber and cannot be done directly without clinical reassessment. Your prescriber will determine the appropriate starting dose for Wegovy based on your individual health profile, not simply by converting your Mounjaro dose.
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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 vs. Private Access for Wegovy in the UK
On the NHS: Wegovy access is through specialist tier 3 weight management services. NICE has approved semaglutide (including Wegovy) within these services, but availability remains limited in many areas due to funding and capacity constraints. Waiting lists can be months or years. If you are currently prescribed Mounjaro via the NHS and wish to switch, discuss this with your specialist team—the decision will be based on your clinical progress, side effects, and suitability for an alternative GLP-1 medication.
Privately: Wegovy is available through online prescribing services without NHS referral. You will have a clinical consultation to confirm eligibility, and if approved, Wegovy is dispensed by a registered pharmacy. Private costs typically range from £129–£320 per month depending on dose. Some people switch between Mounjaro and Wegovy privately due to cost differences at different doses, or side effect tolerance.
- 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 Mounjaro to Wegovy on the NHS?
You can request a switch through your NHS tier 3 specialist weight management service, but the decision is clinical. Your specialist will assess whether switching is appropriate based on your progress, side effects, and suitability. Approval is not assured, and availability depends on your local ICB funding and service capacity.
What is the dose equivalent when switching from Mounjaro to Wegovy?
There is no official direct conversion. Approximate clinical equivalents are: 5 mg Mounjaro ≈ 0.5 mg Wegovy, 10 mg ≈ 1.7 mg, and 15 mg ≈ 2.4 mg. However, your prescriber will assess your individual needs and likely start you on a lower Wegovy dose with gradual titration, rather than direct conversion.
Why might someone switch from Mounjaro to Wegovy?
Common reasons include persistent side effects (such as nausea or gastric upset), different tolerance to the single GLP-1 mechanism versus the dual GIP/GLP-1 approach, cost variations at specific doses, personal preference, or prescriber recommendation based on clinical response. Results vary individually.
Is Wegovy available on the NHS in all UK regions?
No. Wegovy is approved by NICE for NHS specialist services, but actual availability is patchy. Some ICBs fund it readily; others have long waiting lists or limited access. Check with your GP about your local tier 3 weight management service and current Wegovy availability in your area.
How long does it take to switch from Mounjaro to Wegovy?
This depends on your prescriber. You typically stop Mounjaro and, after a washout period (usually 1–2 weeks), begin Wegovy at a starting dose of 0.25 mg, then titrate upward weekly or every few weeks. The full transition process can take 4–6 months to reach an effective maintenance dose.
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