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 Use Mounjaro — Complete UK Injection Guide
Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable weight management medication available in the UK. If your GP or prescriber has recommended Mounjaro, understanding how to use it correctly — from injection technique to dose progression — is essential for safe and effective treatment. This guide explains the practical steps of using Mounjaro and compares it to Wegovy (semaglutide), another popular GLP-1 weight loss injection, so you can understand how both medications work and what to expect from your treatment.
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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.
Mounjaro vs Wegovy — How They Work and Compare
Both Mounjaro and Wegovy are prescription weight loss injections, but they work slightly differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to reduce appetite and support weight loss. Wegovy is a GLP-1-only agonist, activating a single pathway. In clinical trials, Mounjaro users lost an average of 15–21% of body weight (SURMOUNT data), while Wegovy users lost approximately 15% (STEP trial data). Both are once-weekly injections given via a KwikPen (Mounjaro) or auto-injection pen (Wegovy). Mounjaro starts at 2.5 mg and titrates up to a maximum of 15 mg, whereas Wegovy begins at 0.25 mg and reaches a maximum of 2.4 mg. Individual results vary significantly — your prescriber will assess which medication is most suitable for your health profile.
- Mounjaro: Dual mechanism, maximum 15 mg weekly, clinical trial loss ~15–21% body weight
- Wegovy: Single mechanism, maximum 2.4 mg weekly, clinical trial loss ~15% body weight
- Both require NHS or private prescription and clinical assessment
- Both are taken alongside reduced-calorie diet and increased physical activity
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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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How To Inject Mounjaro Safely — Step-by-Step Instructions
Mounjaro comes as a pre-filled KwikPen containing four weekly doses. Inject once every seven days, on the same day each week if possible. Choose an injection site: stomach (most common), upper thigh, or upper arm (requires assistance). Rotate your injection site each week to avoid lipohypertrophy (thickened tissue). Remove the pen from the fridge 15 minutes before injection and inspect it — the medicine should be clear and colourless. Insert the needle at a 90-degree angle, press the injection button slowly until you hear a click, then hold for 3 seconds before removing. Store unopened pens in the fridge at 2–8°C; after first use, you can store at room temperature below 30°C for up to 30 days. If you miss a dose within 4 days, inject as soon as you remember; if more than 4 days have passed, skip that dose and continue your normal schedule.
- 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 difference between Mounjaro and Wegovy?
Mounjaro contains tirzepatide (a dual GIP+GLP-1 agonist) and Wegovy contains semaglutide (GLP-1-only). Mounjaro typically offers slightly higher average weight loss in trials (15–21% vs 15%), but individual results vary. Both are once-weekly injections requiring prescription and clinical assessment. Your prescriber will recommend the most suitable option for your health profile.
How do I inject Mounjaro correctly?
Use a clean injection site (stomach, thigh, or upper arm), insert the needle at 90 degrees, press the button slowly until you hear a click, hold for 3 seconds, then remove. Rotate sites each week. Inject once every seven days on the same day if possible. If unsure, ask your prescriber or pharmacist for a practical demonstration.
What should I do if I miss a Mounjaro dose?
If you remember within 4 days of your missed dose, inject as soon as you can and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and take your next dose on the usual day. Never take a double dose to make up for a missed one.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber — there is no standard conversion chart. Your GP or specialist will assess your progress and decide if a switch is appropriate, then guide the transition carefully. Always inform your prescriber before making any changes.
Which is better — Mounjaro or Wegovy?
Neither is universally "better" — suitability depends on your individual health, medical history, and how your body responds. Clinical trials show Mounjaro achieved slightly higher average weight loss, but results vary widely between people. Your prescriber will assess your eligibility and recommend the medication most likely to be safe and 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.