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 Injection Sites
If you've been prescribed Mounjaro (tirzepatide) in the UK, knowing where to inject is essential for safe and effective use. Mounjaro comes as a pre-filled KwikPen device and can be injected into three main body areas. This guide covers approved injection sites, proper rotation technique, and how Mounjaro injection differs from Wegovy. Always follow your prescriber's instructions and the patient information leaflet that comes with your medication.
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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.
Mounjaro Injection Sites and Proper Rotation
Mounjaro can be injected into three approved areas of your body:
- Stomach (abdomen): The most common injection site. Inject at least 5cm away from your belly button, avoiding areas with bruising, scars, or stretch marks.
- Upper thigh: A practical alternative site. Inject into the outer area of the thigh, where there is adequate soft tissue.
- Upper arm: Available but typically requires assistance from another person to inject safely and accurately.
Site rotation is crucial — inject into a different area each week to reduce the risk of lipohypertrophy (thickened tissue) and ensure consistent absorption. You can rotate between all three sites or focus on rotating within one area. The KwikPen is designed for self-injection into the stomach or thigh; upper arm injections often need help from a healthcare professional or trained assistant.
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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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Mounjaro vs Wegovy: Injection Site Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) use the same three approved injection sites: stomach, thigh, and upper arm. The key difference is the injection device and frequency — Mounjaro is delivered via a pre-filled KwikPen and injected once weekly, whilst Wegovy also uses a pre-filled pen and is injected once weekly. Saxenda, the older GLP-1 option in the UK, requires daily injections, making site rotation more critical. Regardless of which medication you've been prescribed following a clinical assessment, proper site rotation every week and careful technique reduce injection-related side effects like bruising and tissue damage.
- 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. The patient information leaflet and NHS guidance recommend rotating injection sites each week to prevent lipohypertrophy (thickened or lumpy tissue) and ensure consistent medication absorption. You can rotate between stomach, thigh, and upper arm, or rotate within one area.
What should I avoid when choosing an injection site for Mounjaro?
Avoid areas with existing bruising, scars, stretch marks, or skin conditions. Don't inject within 5cm of your belly button if using the stomach site. Avoid areas that are hard, tender, or discoloured. Always check your skin before injecting.
Is the stomach the best area to inject Mounjaro?
The stomach is the most common site because it's easier to access for self-injection, but it's not objectively "best." All three approved sites (stomach, thigh, upper arm) are equally safe when rotated properly. Choose based on comfort, accessibility, and what your prescriber recommends.
Do I need help injecting Mounjaro into my upper arm?
Yes, upper arm injections typically require assistance from another person. If you prefer to self-inject, the stomach or outer thigh are easier and more practical options. Always follow the technique shown in your KwikPen training or patient information video.
Can I use the same injection site if I switch from Mounjaro to Wegovy?
Yes, both medications use the same three approved injection sites (stomach, thigh, upper arm) and the same rotation principle applies. If you switch medications, continue rotating sites weekly to prevent tissue damage and ensure proper absorption of whichever medication you've been prescribed.
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