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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Where to Inject Mounjaro — Best Injection Sites in the UK
Mounjaro (tirzepatide) is injected once weekly under the skin using a KwikPen. The NHS and manufacturer guidance recommend three main injection sites: your stomach, upper thigh, or upper arm. Rotating your injection site each week helps prevent skin irritation and bruising. Understanding the correct technique and location is important for getting the most from your treatment. This guide covers approved UK injection sites, how to rotate them safely, and what to expect when self-injecting.
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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.
Approved Mounjaro Injection Sites in the UK
Mounjaro can be injected into three areas of the body:
- Stomach (abdomen): The most commonly used site. Inject at least 5cm away from your belly button and avoid any scars or bruised areas.
- Upper thigh (front or outer): A convenient alternative, especially if you are self-injecting. Ensure you pinch the skin to create a fold before injecting.
- Upper arm (back of): Usually requires assistance from another person. The prescriber or nurse can show you the correct location and technique.
Each site should be rotated weekly. If you are taking Mounjaro as a UK prescription, your prescriber or pharmacist will provide a step-by-step guide on safe injection technique. Never inject into areas where the skin is tender, bruised, hard, or scarred.
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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.
Rotation Schedule and Safety Tips
Rotating your injection site is essential to prevent lipohypertrophy — a thickening of fatty tissue that can occur if you inject repeatedly in the same spot. A simple rotation pattern is: week 1 stomach, week 2 right thigh, week 3 left thigh, week 4 left arm, then repeat. Always check the skin before injecting; if you notice redness, swelling, or warmth at the last injection site, choose a different location. Store your Mounjaro KwikPen at room temperature (below 30°C) after first use and never inject a cold pen directly from the fridge — allow it to warm to room temperature first for comfort.
- 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.
Can I inject Mounjaro into the same spot every week?
No. Injecting in the same spot repeatedly can cause lipohypertrophy (thickened tissue) and may affect how your body absorbs the medication. Rotate between your stomach, thighs, and upper arms weekly. If you notice bruising, redness, or swelling at a site, skip it and choose a different location.
Does it hurt to inject Mounjaro?
The Mounjaro needle is very fine (31 gauge), so most people report minimal discomfort. Some describe it as a small pinch. Pain varies by individual and injection site. If you find injections uncomfortable, speak to your prescriber or practice nurse — they can provide technique tips to reduce discomfort.
What should I do if I accidentally inject into a scar or bruised area?
If you have injected into a scar or bruised area by mistake, it will not cause harm, but you may experience more bruising or discomfort. For future injections, avoid these areas and rotate to healthy skin. If you have concerns about the injection site, contact your GP or prescriber.
How long does a Mounjaro injection take?
The actual injection takes only a few seconds. However, allow yourself a moment to prepare the pen, check the injection site, and relax. Most people find the entire process takes less than a minute once they are familiar with the routine.
Is there any special care needed after injecting Mounjaro?
After injecting, you do not need any special aftercare. You can continue normal activities immediately. If you notice bruising or mild redness, this is normal and usually resolves within a few days. If you experience severe redness, warmth, or swelling that worsens, contact your GP.
Ready to take control of your weight loss journey?
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