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
Free tracker included with every download this week
Does It Matter Where You Inject Mounjaro?
Yes, injection site does matter for Mounjaro safety and effectiveness. Your Mounjaro KwikPen is designed for use in specific areas of your body, and rotating injection sites is essential to prevent skin problems and ensure consistent absorption. This guide explains approved injection sites, how to rotate them properly, and why location matters for your treatment with this prescription-only tirzepatide injection.
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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 and Rotation
Mounjaro is designed to be injected under the skin (subcutaneously) in three approved areas of your body. Each week, you should rotate between different sites to prevent irritation, bruising, and lipohypertrophy (thickening of fatty tissue at injection sites).
- Stomach (abdomen): The most commonly used site. Inject 5cm away from your belly button and avoid areas that are tender, bruised, red, or hard.
- Upper thigh: A good alternative site. Use the outer area of the thigh for easier access.
- Upper arm (back of the arm): Requires assistance from another person to inject safely. This site is less commonly used but remains an option.
The key principle is never inject in the same spot twice in a row. By rotating sites each week across these three areas, you reduce the risk of injection site reactions, tissue damage, and uneven absorption of your dose. Always ensure your skin is clean and dry before injecting, and never inject into areas that are already irritated or bruised from a previous injection.
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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.
Why Injection Site Matters for Your Mounjaro Treatment
The injection site affects how quickly and evenly your Mounjaro dose enters your bloodstream. Using the same spot repeatedly can lead to lipohypertrophy, where fatty tissue thickens and hardens, making future injections uncomfortable and potentially reducing how well the medication is absorbed. This means your dose may not work as effectively.
Rotating sites also minimises common side effects like redness, bruising, swelling, and pain at the injection site—issues that can make your weekly injection less comfortable. By using a structured rotation pattern (for example: stomach one week, thigh the next, upper arm the week after), you give each area time to recover while maintaining consistent medication delivery. Your prescriber or pharmacist can show you a rotation pattern during your initial consultation or when you collect your first Mounjaro KwikPen prescription.
- 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 any area of my body?
No. Mounjaro is only approved for injection under the skin in three specific areas: the stomach (abdomen), upper thigh, and upper arm. Never inject into other areas like the buttocks, legs, or arms other than the back of the upper arm. Always follow your prescriber's instructions and the guidance in your patient information leaflet.
What happens if I inject Mounjaro in the same spot every week?
Injecting repeatedly in the same site increases the risk of lipohypertrophy (tissue thickening), bruising, pain, and reduced absorption. This can make injections uncomfortable and may affect how well your dose works. Rotating sites prevents these problems and ensures consistent medication delivery.
How should I rotate my Mounjaro injection sites?
Use a simple pattern: stomach one week, upper thigh the next week, then upper arm the following week, then back to stomach. Always inject at least 2–3cm away from your previous injection site. Within each area, move to a different spot each week to avoid irritation and tissue damage.
What should I do if my Mounjaro injection site is bruised or sore?
Avoid injecting into any bruised, red, tender, or hard areas. Choose a different site from your approved areas. If soreness or bruising persists beyond a few days, or if you develop signs of infection (increasing redness, warmth, pus), contact your GP or pharmacist for advice.
Does the injection site affect how well Mounjaro works?
Yes. Rotating sites ensures consistent and efficient absorption of your tirzepatide dose. Repeated injections in the same spot can cause tissue changes that reduce absorption, potentially making your medication less effective. Proper site rotation helps you get the full benefit of your prescribed dose.
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.