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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Mounjaro Pen 5mg vs Wegovy — How They Compare
Both Mounjaro and Wegovy are prescription-only weight management injections available in the UK, but they work differently and produce different results. Mounjaro (tirzepatide) uses a dual GIP and GLP-1 mechanism, whilst Wegovy (semaglutide) activates GLP-1 alone. Understanding the differences can help you and your prescriber decide which might be right for you. Both require clinical assessment before prescription.
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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.
Clinical Trial Results and How They Differ
Mounjaro (SURMOUNT trial): At the 5mg dose (the first active maintenance dose), clinical trials showed participants lost an average of approximately 15% of their body weight over 72 weeks, alongside reduced-calorie diet and increased physical activity. Higher doses (10mg and 15mg) showed greater average losses — up to around 21% at the maximum dose.
Wegovy (STEP-1 trial): At the maximum 2.4mg dose, participants achieved an average weight loss of approximately 15% over 68 weeks under the same lifestyle conditions.
Key point: At equivalent doses, Mounjaro showed slightly greater average weight loss in trials, but individual results vary significantly. Trial averages do not predict personal outcomes. Use our free GLP-1 weight loss calculator to explore potential loss projections at different doses.
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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.
Mechanism, Dosing, and How to Access in the UK
How they work: Mounjaro is a dual GIP/GLP-1 agonist — it activates two appetite-regulating pathways. Wegovy is GLP-1 only. This dual mechanism may explain Mounjaro's slightly higher average weight loss in trials, though both medications slow gastric emptying and reduce hunger.
Dosing and access: Mounjaro starts at 2.5mg, titrating to 5mg, then escalating weekly up to 15mg maximum. Wegovy starts at 0.25mg, titrating up to 2.4mg maximum. Both are available through NHS specialist weight management services (with significant regional variation and waiting lists) or via private online prescription following clinical assessment with a UK-licensed prescriber. Your GP or prescriber will assess eligibility and suitability based on your health.
---COL_J_ALT---Side effects: Both share common side effects including nausea, diarrhoea, constipation, and stomach pain — typically most pronounced in the first 2–4 weeks and often improving with continued use. Mounjaro users report slightly higher initial nausea in trials. Hair loss is reported more frequently with Mounjaro. Both require stopping 4 weeks before conception due to effects on oral contraceptive absorption.
Cost and availability: Private prescription costs range from approximately £129–£320/month depending on dose. NHS access is through tier 3 specialist services where available. Availability varies by ICB region — some areas have 12+ month waiting lists, others offer faster access.
- 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.
Is Mounjaro 5mg better than Wegovy?
"Better" depends on individual factors. Mounjaro 5mg showed slightly higher average weight loss than Wegovy's maximum dose in trials (15% vs 15% at equivalent strength, with Mounjaro showing more at higher doses), but results vary greatly. Your prescriber will assess which suits your health, medical history, and preferences. Both are effective for many people.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible, but it must be managed by your prescriber. There is no official conversion chart — approximate equivalence suggests Wegovy 0.5mg ≈ Mounjaro 5mg, but any switch requires clinical guidance. You cannot simply stop one and start the other without medical advice.
Which injection is easier to use — Mounjaro KwikPen or Wegovy?
Both use pre-filled pens and are administered once weekly. Mounjaro uses the KwikPen auto-injection device. Wegovy uses an auto-injection pen. Both are designed to be user-friendly. Choice between them is typically based on medical suitability, not ease of use.
How long until I see results with Mounjaro 5mg?
Individual timelines vary. Most people notice appetite reduction within 3–7 days of starting or increasing dose. Measurable weight loss typically appears within 2–4 weeks. Clinical trials measured outcomes over 68–72 weeks. Track your progress with our free progress tracker to monitor changes over time.
Do I need a prescription to buy Mounjaro or Wegovy in the UK?
Yes, both are prescription-only medicines in the UK. You cannot purchase either without a valid UK prescription from a licensed prescriber (GP via NHS services, or via private online clinical assessment). Dispensing is through registered GPhC pharmacies only.
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.