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 vs Wegovy: The Weight Loss Pen Tirzepatide Explained
Mounjaro and Wegovy are both prescription weight loss injections available in the UK, but they work differently. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist, whilst Wegovy contains semaglutide, a GLP-1 receptor agonist only. This fundamental difference in mechanism affects how they work, their dosing schedules, and the results seen in clinical trials. Understanding these differences helps you and your GP make informed decisions about which prescription weight loss injection might be appropriate for you. Both are available through NHS specialist services or private prescribing following clinical 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.
How Tirzepatide and Semaglutide Work Differently
Tirzepatide (Mounjaro) is a dual GIP receptor and GLP-1 receptor agonist. This means it activates two natural hormone pathways in your body. The GIP receptor helps regulate appetite and glucose control, whilst the GLP-1 receptor signals fullness and reduces hunger cravings. By targeting both pathways simultaneously, tirzepatide may provide a more comprehensive approach to appetite regulation.
Semaglutide (Wegovy) is a GLP-1 receptor agonist only, meaning it works through a single hormone pathway. It signals your brain that you are full, slows stomach emptying, and reduces food cravings—but does not activate the GIP pathway that tirzepatide targets.
This dual-mechanism approach of tirzepatide may explain why some clinical trial data suggests slightly higher average weight loss with Mounjaro compared to Wegovy, though individual results vary significantly.
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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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Doses, Eligibility, and NHS Access in the UK
Mounjaro (tirzepatide) starts at 2.5 mg weekly, escalating up to 15 mg maximum. Wegovy (semaglutide) starts at 0.25 mg weekly, escalating to 2.4 mg maximum. Both are weekly weight loss injection options available as a pre-filled KwikPen or auto-injector pen.
In the UK, both prescription weight loss medications are available through NHS tier 3 specialist weight management services or private online prescribing. Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes or cardiovascular disease. Your GP or a licensed prescriber will assess whether either medication is clinically appropriate for you following a full health review.
- 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 is the main difference between Mounjaro and Wegovy?
Mounjaro contains tirzepatide, which activates both GIP and GLP-1 receptor pathways. Wegovy contains semaglutide, which targets GLP-1 only. This dual mechanism in tirzepatide may explain why some clinical trials showed slightly higher average weight loss with Mounjaro, though individual results vary considerably.
How much weight loss can I expect from tirzepatide?
Clinical trials (SURMOUNT-1) showed that at the maximum 15 mg dose, tirzepatide resulted in average weight loss of approximately 20.9% of body weight over 72 weeks, alongside a reduced-calorie diet and physical activity. Individual results vary—some people lose more, others less. Your prescriber can discuss realistic outcomes for your situation.
Can I switch from Wegovy to Mounjaro?
A rough equivalence exists (10 mg Mounjaro ≈ 1.7 mg Wegovy), but any medication switch must be managed by your prescriber. They will assess whether switching is appropriate, manage the transition safely, and monitor your response to the new medication.
How do I access tirzepatide (Mounjaro) in the UK?
Through NHS specialist weight management services (tier 3) following referral from your GP—waiting lists vary by region. Or through private online prescribing with a UK-licensed prescriber after clinical assessment. Both routes require a prescription and individual eligibility assessment.
Are there other weight loss injections available in the UK?
Yes. Saxenda (liraglutide) is a daily GLP-1 injection. Mounjaro and Wegovy are both once-weekly options. All three are prescription-only and licensed for weight management in UK adults meeting eligibility criteria. Your GP or prescriber can discuss which might suit your lifestyle and health profile.
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.