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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Moving From Mounjaro to Wegovy: What You Need to Know
If you're considering switching from Mounjaro (tirzepatide) to Wegovy (semaglutide), the decision should always be made in consultation with your prescriber. Both are prescription-only weight management injections licensed in the UK, but they work through different mechanisms and require individualised clinical assessment. This guide explains the NHS context, approximate dose equivalence, and what to expect during a medication switch.
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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 Eligibility and Prescribing Context for Both Medications
Both Mounjaro and Wegovy are available through NHS specialist weight management services (tier 3) and private prescribing in the UK. Under NICE guidance, both medications are approved for adults with a BMI of 30 kg/m² or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea. For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply due to higher cardiovascular risk at these weights.
Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 receptor agonist only. This difference in mechanism means they are not directly interchangeable — a switch requires your prescriber's assessment. STEP clinical trial data showed Wegovy achieved approximately 15% average body weight loss, whilst SURMOUNT trial data showed Mounjaro achieved 15–21% loss depending on dose. Individual results vary significantly.
- Mounjaro: maximum dose 15 mg once weekly, taken via KwikPen
- Wegovy: maximum dose 2.4 mg once weekly, taken via pre-filled auto-injection pen
- Both require a clinical assessment before prescription
- Both are subject to ongoing MHRA monitoring (Black Triangle ▼ status)
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS vs Private Route: How to Access Wegovy in the UK
NHS access: Wegovy is available through NHS tier 3 specialist weight management services. You'll need a GP referral and assessment. Availability and waiting times vary significantly by region — some ICBs (Integrated Care Boards) have long waiting lists, whilst others offer swifter access. Your GP can advise on local availability.
Private access: Wegovy is available through UK-registered online prescribing services. You'll need to complete an online clinical assessment with a UK-licensed prescriber. If approved, a prescription is issued and the medication is dispensed by a GPhC-registered pharmacy. This route is faster but involves a cost, typically ranging from £129–£320 per month depending on dose.
Use our GLP-1 weight loss calculator to compare potential weight loss outcomes, and our BMI calculator to confirm your current eligibility for either medication.
- 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.
What's the approximate dose equivalent if I switch from Mounjaro to Wegovy?
Approximate equivalence: Mounjaro 5 mg ≈ Wegovy 0.5 mg; Mounjaro 10 mg ≈ Wegovy 1.7 mg; Mounjaro 15 mg ≈ Wegovy 2.4 mg. These are clinical practice estimates only — there's no official conversion chart. Your prescriber will assess which Wegovy dose is appropriate for you based on your response to Mounjaro and individual health factors. Never adjust doses without prescriber guidance.
Will I experience the same side effects switching from Mounjaro to Wegovy?
Side effect profiles are similar (nausea, diarrhoea, vomiting, constipation, stomach pain are common), but individual responses vary. Some people tolerate one medication better than the other. If you've had troublesome side effects on Mounjaro, discuss this with your prescriber before switching — they may adjust your starting dose on Wegovy or consider remaining on Mounjaro with optimised management.
Do I need a new NHS referral if I want to switch from Mounjaro to Wegovy on the NHS?
If you're already under NHS tier 3 weight management services, your specialist team can manage a switch to Wegovy without a new referral. If you're on private Mounjaro and want NHS Wegovy, you'll need a new GP referral to NHS services. This may involve a waiting period depending on your region's current capacity.
How long does it take to switch medications?
Your prescriber will typically stop Mounjaro after your final dose and start Wegovy after a brief interval (usually 3–7 days) to allow time between injections. Do not overlap doses. The transition period should be explained clearly by your prescriber before you begin — it's important to understand the timing to avoid gaps in treatment.
What if Wegovy doesn't work as well as Mounjaro for me?
If you find Wegovy less effective, inform your prescriber immediately. You may return to Mounjaro, switch to another medication, or adjust your dose. Weight loss medication effectiveness varies individually — what works well for one person may need optimisation for another. Your prescriber will work with you to find the best option.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.