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 Explained
Mounjaro (tirzepatide) comes as a once-weekly weight loss injection in the UK, and choosing the right injection site is important for effectiveness and comfort. The approved sites for your weekly weight loss injection are your stomach (abdomen), upper thigh, or upper arm — with specific techniques recommended to rotate between them. This guide explains the best Mounjaro injection sites, how to use your KwikPen safely, and what NHS guidance says about injection technique and site care.
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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 and How to Use Them
Mounjaro comes in a pre-filled KwikPen device with four doses per pen (one month's supply). The MHRA-approved injection sites for Mounjaro are:
- Stomach (abdomen) — 5cm away from your belly button. This is the most commonly used site and generally easiest to self-inject.
- Upper thigh — Front or outer surface, at least 2.5cm from any previous injection site. Good for rotation.
- Upper arm — Back of the upper arm (requires help from another person to inject safely).
The key to safe Mounjaro use is rotating your injection site each week. Never inject into the same spot twice in a row — this reduces injection site reactions and helps your body absorb the medication evenly. If you're injecting into the same area (e.g., always stomach), alternate between left and right sides, or move up and down slightly. Wait at least one week before returning to the exact same spot.
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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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Injection Technique, Site Care, and What to Expect
Before injecting, check your KwikPen hasn't been frozen and allow it to reach room temperature if stored in the fridge. Clean the injection site with an alcohol wipe and let it dry for a few seconds. Pinch the skin gently, insert the needle at a 90-degree angle, and hold the pen steady for 3 seconds while injecting. After injection, you may notice slight redness, bruising, or tenderness — this is normal and usually settles within a few hours. If you experience persistent pain, severe redness, or signs of infection, contact your GP. Our GLP-1 weight loss calculator can help you track your progress across different dose levels as you follow your titration schedule.
- 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. You must rotate your injection site each week to reduce the risk of injection site reactions, lipohypertrophy (thickened tissue), and uneven absorption. Within the same area (e.g., stomach), alternate sides or move the injection point at least 2.5cm away from the previous site.
What if I inject Mounjaro into muscle instead of subcutaneous tissue?
Mounjaro must be injected into the layer of fat under the skin (subcutaneous), not into muscle. Injecting into muscle can cause pain and may affect how the medication works. Always pinch the skin and inject at a 90-degree angle into the fat layer. If you're unsure, ask your prescriber or pharmacist for a demonstration.
What should I do if my Mounjaro injection site is red or painful?
Mild redness and slight bruising are normal and usually resolve within hours. If redness is severe, spreading, warm to touch, or you develop signs of infection (pus, warmth, fever), contact your GP immediately. This could indicate an infection requiring treatment.
Can I inject Mounjaro through clothing or over scar tissue?
Always inject into clean, bare skin. Never inject through clothing. Avoid scar tissue, moles, and areas with bruising or hardened tissue from previous injections. Choose a fresh area each week to ensure proper absorption and reduce complications.
Is there NHS guidance on Mounjaro injection sites?
Yes. NHS specialist weight management services and the MHRA-approved prescribing information specify the three approved sites: stomach, upper thigh, and upper arm. Your prescriber or pharmacist should provide detailed injection training when you start Mounjaro on the NHS or privately.
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.