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 Sites and Safety Tips
Choosing the right injection site for your weekly weight loss injection is important for comfort and effectiveness. Mounjaro (tirzepatide) can be injected into three main areas of your body, and rotating between sites helps prevent irritation and ensures consistent absorption. This guide explains the best Mounjaro injection sites, how to rotate correctly, and how injection administration compares between Mounjaro and Wegovy — both prescription-only GLP-1 and GIP-based weight loss medications available in the UK.
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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 — Where and How to Inject
Mounjaro comes as a pre-filled KwikPen device that delivers a once-weekly injection. The three approved injection sites are:
- Stomach (abdomen) — the most common and easiest site for self-injection, using the area around your belly but avoiding the navel itself
- Upper thigh — a convenient alternative that many people rotate to weekly, accessed on the front or outer area of the thigh
- Upper arm — the back of the upper arm, though this typically requires help from another person to inject safely
NHS guidance and Mounjaro's product information recommend rotating your injection site each week — this means if you inject in your stomach one week, switch to your thigh the next week, then your arm the following week, then back to stomach. This rotation reduces the risk of lipohypertrophy (thickened skin), bruising, and injection site reactions. Always use a fresh needle for each dose and inject at least 2.5 cm away from your previous injection site.
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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 Administration and Mechanisms
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work slightly differently. Wegovy (semaglutide) is a GLP-1 receptor agonist, whilst Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways instead of one. Both are administered as once-weekly injections using pre-filled pen devices, and both use the same three injection sites (stomach, thigh, upper arm). Clinical trials showed Mounjaro resulted in approximately 15–clinically studied weight loss, whilst Wegovy showed around 14.9% average weight loss. Injection site reactions, bruising, and pain are common with both medications but typically mild. Your prescriber will assess which medication is more suitable for you based on your individual health profile and medical history.
- 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's the best Mounjaro injection site for beginners?
The stomach (abdomen) is generally the easiest and most comfortable site for most people new to injecting Mounjaro. The area is easier to access and inject yourself without assistance. Avoid injecting directly into your navel, and always rotate sites weekly to prevent skin thickening and irritation.
Can I use the same injection site every week?
No — rotating sites weekly is important. Injecting in the same area repeatedly can cause lipohypertrophy (thickened or lumpy skin) and may affect how the medication is absorbed. The NHS and Mounjaro guidance recommend switching between stomach, thigh, and upper arm each week.
How far apart should Mounjaro injection sites be?
You should inject at least 2.5 centimetres away from your previous injection site. This spacing helps prevent skin damage, bruising, and irritation. Keep a mental note of where you injected the week before to ensure adequate distance.
Is Wegovy injected in the same places as Mounjaro?
Yes, both Mounjaro and Wegovy use the same three injection sites: stomach, upper thigh, and upper arm. Both are once-weekly injections administered via pre-filled pen devices. Site rotation guidance is the same for both medications.
What should I do if my Mounjaro injection site becomes red or painful?
Mild redness and soreness at the injection site are normal and usually resolve within a few hours. However, if you develop severe pain, significant swelling, signs of infection, or if symptoms persist beyond 48 hours, contact your GP or the prescriber who issued your prescription for advice.
Ready to take control of your weight loss journey?
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