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 Should You Inject Mounjaro? Best Sites Explained
Mounjaro (tirzepatide) comes as a pre-filled KwikPen injection that you'll administer once weekly at home. The medicine works the same way regardless of where you inject it, but choosing the right injection sites and rotating between them properly helps prevent skin irritation and ensures consistent absorption. This guide covers the three approved injection areas, how to rotate sites correctly, and practical tips for getting the most from your tirzepatide prescription.
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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 in the UK
The UK prescribing information for Mounjaro (tirzepatide) specifies three safe injection sites where you can administer your weekly dose. These areas have enough subcutaneous tissue to safely deliver the medicine and minimise discomfort.
- Stomach (abdomen) — Most common choice. Inject into the soft tissue on either side of your navel, avoiding the area directly around it. This site is convenient and many people find it easiest to access.
- Upper thigh (front or outer) — Good alternative site. You can inject yourself or have someone help. Provides a different area for rotation.
- Upper arm (back of the upper arm) — Valid injection site but typically requires assistance, as it's difficult to reach and inject yourself safely.
All three areas are equally effective for delivering tirzepatide. The medicine absorbs similarly from each site. Rotating between sites every week helps prevent lipohypertrophy (thickened fatty tissue) and ensures your skin stays healthy. Your prescriber or pharmacist can demonstrate the correct injection technique when you first start.
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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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How to Rotate Sites and Prevent Injection Problems
Proper injection site rotation is key to safe, comfortable Mounjaro use. Each week, choose a different location within your selected area or switch to a completely different body area. For example, inject into the right side of your stomach one week, then the left side the next week, then your thigh the week after.
Why rotation matters: Repeated injections in the same spot can cause lipohypertrophy (hard, lumpy patches of tissue), which may affect how well the medicine absorbs. Rotation spreads injections across healthy tissue and reduces this risk. If you notice any lumps, bruising, redness, or persistent pain at an injection site, inform your GP or prescriber immediately — these may indicate a local reaction requiring a different approach.
Always use a new needle for each injection. Before injecting, check that the Mounjaro solution is clear and colourless. Never inject into areas where the skin is broken, bruised, or tender.
Storage reminder: Store unopened Mounjaro pens in the fridge at 2–8°C. After first use, you can keep the pen at room temperature (below 30°C) for up to 30 days. Always check the expiry date on your KwikPen before injecting. If you've stored a pen incorrectly or it has been frozen, do not use it — consult your pharmacist about a replacement.
- 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.
Does the injection site affect how well Mounjaro works?
No. Tirzepatide works the same way regardless of whether you inject into your stomach, thigh, or upper arm. All three approved sites have sufficient subcutaneous tissue for proper absorption. Individual variation in skin thickness may cause minor differences in absorption speed, but clinical effectiveness is consistent across all three areas.
Can I inject Mounjaro into the same spot every week?
It's not recommended. Repeated injections in the same spot increase the risk of lipohypertrophy (thickened fatty tissue patches) and local irritation. Rotate between different sites each week — for example, alternate between your left and right stomach sides, then use your thigh, then your arm. This keeps your skin healthy and maintains consistent absorption.
What should I do if I get pain, bruising, or lumps at an injection site?
Mild bruising and soreness are normal after any injection. However, if you develop persistent pain, hard lumps, significant redness, warmth, or swelling that doesn't improve within a few days, contact your GP or prescriber. These could indicate lipohypertrophy, infection, or a local allergic reaction. They may recommend changing your injection technique or sites.
Does temperature affect Mounjaro before I inject?
Yes. Store unopened pens in the fridge at 2–8°C. Once opened, you can keep the pen at room temperature (below 30°C) for up to 30 days. Never freeze Mounjaro — if a pen freezes, discard it. Allow a cold pen to reach room temperature for a few minutes before injecting, as injecting very cold medicine can increase discomfort.
Who should I contact if I'm unsure about my injection technique?
Your prescriber, GP, or pharmacist can demonstrate the correct technique when you start Mounjaro. Most GPhC-registered online pharmacies provide injection training videos or written guides with your first prescription. If you're uncertain at any point, contact your prescriber or pharmacist — they're there to help you use tirzepatide safely and confidently.
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