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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GLP-1 vs Mounjaro: What's the Difference?
GLP-1 and Mounjaro are both prescription-only weight loss injections available in the UK, but they work differently. GLP-1 medications like Wegovy activate one hormone pathway, whilst Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two pathways. Both are licensed for weight management in UK adults with a BMI of 30 or above, or 27 and above with weight-related health conditions. Understanding how they differ helps you and your GP make an informed decision about which might be suitable for you.
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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 GLP-1 and Mounjaro Work Differently
GLP-1 receptor agonists like Wegovy activate the GLP-1 hormone pathway. This signal tells your brain you're full, slows food leaving your stomach, and helps regulate blood sugar. It's a single mechanism of action.
Mounjaro (tirzepatide) is different — it activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 pathways simultaneously. This dual action may provide stronger appetite suppression and enhanced weight loss support compared to GLP-1 alone. Clinical trial data showed Mounjaro helped participants lose an average of 15–21% of body weight, whilst GLP-1 medications like Wegovy showed approximately 15% average weight loss. However, individual results vary significantly.
Both are administered as once-weekly injections using a pre-filled KwikPen or auto-injection pen, and both require a prescription and clinical assessment before use.
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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 Eligibility and Prescribing Pathway
In the UK, both GLP-1 and Mounjaro are available through NHS specialist tier 3 weight management services or privately. NHS eligibility criteria are identical for both: you may qualify if you have a BMI of 30 kg/m² or above, or 25–29.9 with weight-related health conditions such as type 2 diabetes, prediabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, eligibility may start at a lower BMI threshold due to higher cardiovascular risk. NICE (National Institute for Health and Care Excellence) has approved tirzepatide (Mounjaro) for NHS use within specialist services. Access varies by region and waiting lists can be significant. Your GP can refer you to your local weight management service for assessment.
- 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 a GLP-1 medication?
Mounjaro is not a traditional GLP-1 medication — it's a dual GIP and GLP-1 receptor agonist. It activates two hormone pathways instead of one, which is why it's classified separately from GLP-1 medications like Wegovy. Both are weight loss injections available in the UK, but the mechanism differs.
Which causes more side effects, GLP-1 or Mounjaro?
Side effect profiles are similar. Both commonly cause nausea, diarrhoea, and constipation, especially during the first few weeks or when increasing doses. Individual tolerability varies. Your prescriber can discuss which is more likely to suit your health profile during assessment.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications is possible but must be managed by your prescriber. If you're receiving Wegovy through the NHS and it's not working well, discuss with your weight management service team. They can assess whether Mounjaro might be more appropriate for you.
What dose of Mounjaro is used for weight loss in the UK?
Mounjaro starts at 2.5 mg once weekly and increases gradually to a maintenance dose, typically between 5 mg and 15 mg depending on response and tolerability. The KwikPen device comes pre-loaded with your prescribed dose. Your prescriber determines your personalised dose schedule.
How do I access these injections — NHS or private?
NHS access requires a GP referral to your local tier 3 specialist weight management service. Private access is available through online consultations with UK-licensed prescribers, without needing NHS referral. Both routes require clinical assessment. Waiting times and costs differ significantly between NHS and private routes.
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.