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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How to Do Mounjaro — UK Injection Guide
Mounjaro (tirzepatide) is a once-weekly prescription injection that requires proper technique to get the most from your treatment. If you've been prescribed Mounjaro or are considering it, understanding the injection process, dose titration schedule, and how it compares to Wegovy can help you feel confident about your weight management journey. This guide covers everything you need to know about using Mounjaro safely and effectively in the UK, alongside how it differs from other GLP-1 weight loss injections.
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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 Mounjaro Injections Work and Compare to Wegovy
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it works on two hormone pathways rather than one. Wegovy (semaglutide) is a GLP-1 agonist only. This difference explains why clinical trial results vary between the two medications.
SURMOUNT trial data showed Mounjaro users lost an average of 15–21% of their body weight depending on dose, whilst STEP trial data for Wegovy showed approximately 15% average weight loss. Both are taken once weekly as injections using pre-filled pens.
- Mounjaro KwikPen: Doses from 2.5 mg up to 15 mg, each pen contains four weekly injections
- Wegovy pen: Doses from 0.25 mg up to 2.4 mg, four doses per pen
- Starting doses: Mounjaro begins at 2.5 mg; Wegovy at 0.25 mg
Both medications require a UK prescription from a licensed prescriber following a clinical assessment. Individual results vary significantly — your GP or prescriber will determine which is suitable based on your personal health profile and medical history.
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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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Step-by-Step: How to Inject Mounjaro and Suitability Factors
When you receive your Mounjaro KwikPen, your prescriber or pharmacist will demonstrate the correct injection technique. Inject once weekly on the same day each week into your stomach (abdomen), upper thigh, or upper arm — rotating sites each week to prevent irritation. The pen has a safety mechanism that clicks when the dose is delivered, so you know the injection is complete.
Both Mounjaro and Wegovy require UK prescription and clinical assessment — neither can be obtained without a prescription from a licensed healthcare professional. Your prescriber will assess your eligibility based on BMI, weight-related health conditions, and current medications. Try our free GLP-1 weight loss calculator to estimate potential weight loss outcomes, and use our BMI calculator to understand your starting point and healthy weight range for your height.
- 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 main difference between how Mounjaro and Wegovy work?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only, working on a single pathway. This dual mechanism may explain why some clinical trial participants lost slightly more weight on Mounjaro, but individual results vary significantly.
How do I inject Mounjaro correctly?
Inject once weekly into your stomach, thigh, or upper arm using the KwikPen. Rotate injection sites each week. Your prescriber or pharmacist will demonstrate the correct technique before your first injection. You'll hear and feel a click when the dose is delivered. Store unopened pens in the fridge at 2–8°C.
Which is better for weight loss — Mounjaro or Wegovy?
Both are effective prescription weight loss medications. Clinical trials showed slightly higher average weight loss with Mounjaro (15–21%) versus Wegovy (approximately 15%), but this varies between individuals. Your prescriber will recommend the most suitable option based on your health profile, not on which showed higher average results in trials.
Can I get Mounjaro on the NHS in the UK?
NICE has approved tirzepatide (Mounjaro) for NHS specialist weight management services, but access varies by region. Many people access it privately through UK-licensed prescribers following an online clinical assessment. Your GP can advise on NHS availability in your area.
How long does a Mounjaro pen last?
Each KwikPen contains four weekly doses, so one pen lasts four weeks. As you increase your dose during the first 5–20 weeks, you'll use multiple pens. Once you reach your maintenance dose, you continue with weekly injections indefinitely while the medication remains effective for you.
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.