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 Tirzepatide: Safe Injection Sites
If you've been prescribed tirzepatide (Mounjaro) as a weekly weight loss injection in the UK, knowing the correct injection sites is essential for safe and effective treatment. Tirzepatide is administered subcutaneously — just under the skin — and there are several approved areas on your body where you can inject. This guide explains each site, how to rotate injections, and what to expect during administration.
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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 Tirzepatide Injection Sites
Tirzepatide comes in a pre-filled KwikPen device and can be injected into three main areas of your body:
- Stomach (abdomen): The most commonly used site. Inject into the area around your navel, avoiding the skin directly on or within 5cm of your belly button. This area has good blood flow and is easy to access.
- Upper thigh (anterior or outer thigh): A practical alternative if you prefer not to inject your stomach. Pinch the skin and inject into the thickest part of your thigh, avoiding the inner thigh area.
- Upper arm (back of the arm): Another approved site, though it may require assistance to inject safely. Inject into the back of your upper arm, roughly halfway between shoulder and elbow.
All three sites are approved for tirzepatide in the UK. Your GP or prescriber will confirm which sites are suitable for you. The GLP-1 receptor agonist in Mounjaro works effectively from any of these subcutaneous locations.
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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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Rotating Injection Sites and Key Safety Tips
Rotation is important: Change your injection site each week. If you inject your stomach on week one, use your thigh on week two, then your arm on week three, before returning to your stomach. This reduces the risk of lipohypertrophy (thickened fat deposits) and ensures consistent medicine absorption.
Before injecting, wash your hands and clean the injection site with an alcohol wipe. Allow the skin to dry completely. Never inject into areas that are bruised, tender, or scarred from previous injections. If you notice redness, swelling, or pain at an injection site that doesn't resolve within a few days, contact your GP. Store your KwikPen according to guidance — refrigerated before first use, then at room temperature for up to 30 days after opening.
- 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 tirzepatide into the same spot every week?
No. Always rotate injection sites weekly to prevent lipohypertrophy (thickened fat deposits) and ensure consistent absorption. Rotate between stomach, thigh, and upper arm systematically.
Does it hurt to inject tirzepatide?
Most people report minimal discomfort. The needle is very fine (31-gauge). Anxiety before injection is common but usually subsides after the first few doses. If injection pain is severe or persistent, contact your GP.
What should I do if I've injected tirzepatide incorrectly?
If you believe you've made an error (intramuscular injection instead of subcutaneous, for example), contact your GP or call NHS 111. Do not attempt to re-inject the same dose. Your prescriber can advise on your next step.
Is tirzepatide available on the NHS for weight loss?
Yes. NICE has approved tirzepatide (Mounjaro) for weight management in eligible UK adults. Access is through NHS specialist tier 3 weight management services following a clinical assessment. Eligibility typically requires BMI ≥30, or BMI 27–29.9 with weight-related health conditions.
Can I self-inject tirzepatide, or does a nurse have to do it?
You can self-inject tirzepatide once trained by a healthcare professional. Most people inject themselves at home weekly using the pre-filled KwikPen. Your GP practice or prescriber will provide full training before your first injection.
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