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 — Safe Sites and Rotation Guide
Mounjaro (tirzepatide) is a once-weekly weight loss injection administered as a subcutaneous injection — meaning the needle goes just under the skin rather than into muscle or vein. In the UK, there are three approved injection sites you can use, and rotating between them is important to prevent skin irritation and ensure consistent medication absorption. This guide explains exactly where and how to inject Mounjaro safely, based on UK clinical practice guidance.
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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.
Approved Mounjaro Injection Sites
Mounjaro comes as a pre-filled KwikPen device and can be injected into three specific areas of your body:
- Stomach (abdomen): The most commonly used site. Inject into the fatty tissue of your belly, avoiding the area 5cm (2 inches) around your navel. This site tends to have good absorption and is easiest to access yourself.
- Upper thigh: The outer area of your thigh is an approved site. Pinch the skin slightly and inject into the fatty tissue. This site works well and is accessible, though some people find it less convenient than the abdomen.
- Upper arm: The back of your upper arm (triceps area) is an approved site, but it usually requires assistance from another person to inject safely and accurately. This site is less commonly used than the abdomen or thigh.
Your prescriber or pharmacist will demonstrate the correct injection technique during your initial consultation or when your prescription is dispensed.
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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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Rotating Injection Sites and Best Practice
Site rotation is essential. Inject into a different site each week to prevent lipohypertrophy (lumpy tissue build-up) and lipoatrophy (loss of fatty tissue). Within each site, you can also vary the exact location — for example, if you inject into your left abdomen one week, inject your right abdomen the next week.
Always check your skin before injecting: avoid areas with bruising, scarring, hardened tissue, or active irritation. Never inject into areas with lumps or nodules. If you notice persistent skin changes at injection sites, inform your GP or prescriber. Keep the KwikPen at room temperature (below 30°C) and never inject a frozen or extremely cold pen — allow it to reach room temperature first.
Common Questions About Mounjaro Injection Sites- 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 site every week?
No. Rotating between different injection sites each week is important to prevent skin complications like lipohypertrophy (lumpy tissue) and ensure consistent absorption. Use your abdomen one week, thigh the next, then abdomen again the following week — varying the exact location within each site as well.
Is the stomach the best injection site for Mounjaro?
The abdomen is the most commonly used site because it's easy to access, has good medication absorption, and most people find it convenient. However, the thigh is equally effective. Choose based on your comfort and convenience, but always rotate sites weekly.
Does injecting Mounjaro hurt?
Most people report minimal pain — the needle is small and designed for subcutaneous injection. Some describe a slight pinch. If you experience sharp pain or hit a nerve, stop and choose a different spot. Pain usually decreases with practice as you become more confident with the injection technique.
What should I do if I notice lumps or hardness at my injection sites?
Lumps or hardened areas suggest lipohypertrophy. Stop injecting in that area immediately and rotate to a different site. Inform your GP or prescriber, as they may need to assess the area. Continue rotating properly to prevent this developing further.
Can I use the same spot within my abdomen if I inject on different days?
You should rotate between different areas and sites, but if you must use the abdomen weekly, vary the exact location each time — inject upper left one week, lower right the next, etc. This helps prevent tissue damage and ensures better medication absorption over time.
Ready to take control of your weight loss journey?
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