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
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. There are three approved injection sites on your body, and rotating between them is important to prevent complications. Your GP or prescriber will show you the correct injection technique, but understanding the recommended sites helps you use your medication safely and effectively. This guide covers NHS guidance on Mounjaro injection sites and best practices for self-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 Mounjaro Injection Sites
Mounjaro can be injected into three main areas of your body:
- Stomach (abdomen) — the most common injection site, approximately 5cm away from your navel and above the hip bone
- Upper thigh — the outer surface of the thigh, roughly midway between hip and knee
- Upper arm (back of the arm) — the fatty tissue on the back of your upper arm, below the shoulder
Each area provides good subcutaneous (under the skin) absorption. Rotating your injection site each week is essential to prevent lipohypertrophy (thickened fatty tissue) and lipoatrophy (loss of fatty tissue), which can affect how well the medication absorbs. The NHS recommends choosing a different site for each weekly injection — for example, week 1 stomach, week 2 thigh, week 3 arm, then repeat the cycle. Always ensure the site is clean and dry before injecting, and allow the KwikPen to reach room temperature for 30 minutes before use.
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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 and Important Safety Points
When you first start tirzepatide, your prescriber will demonstrate the correct injection technique. Key safety points include: using a fresh needle each time, pinching the skin gently to create a fold, inserting the needle at a 90-degree angle, and holding it in place for approximately 10 seconds before withdrawing. Never inject into areas with bruising, scarring, or hardened tissue. If you experience pain, swelling, or redness at the injection site that persists beyond a few days, contact your GP. Store your KwikPen in the refrigerator (2–8°C) when unopened, and keep it at room temperature (below 30°C) once in use, disposing of it after 30 days. Our GLP-1 weight loss calculator can help you understand potential outcomes alongside proper injection technique and lifestyle changes.
- 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. Rotating injection sites weekly is important to prevent lipohypertrophy and ensure consistent absorption. Use a different site (stomach, thigh, or arm) each week. Always wait at least one week before injecting into the same site again.
Is the stomach the best site to inject Mounjaro?
The stomach is the most commonly used site and offers reliable absorption, but all three approved sites (stomach, thigh, and arm) work equally well. The best site is whichever you can rotate consistently — variety matters more than location.
What should I do if my Mounjaro injection site becomes red or swollen?
Mild redness and swelling usually resolves within a few hours. However, if redness, swelling, or pain persists beyond 24 hours, becomes severe, or shows signs of infection, contact your GP. Avoid using that site until it has fully healed.
Can I inject Mounjaro through clothing?
No. Always inject into clean, dry, bare skin. Injecting through clothing reduces injection accuracy and increases infection risk. Wash and dry the area with an alcohol wipe if available, wait for it to dry, then inject.
Do I need medical supervision to self-inject Mounjaro?
Your prescriber will demonstrate the technique during your first prescription, but you can self-inject at home once trained. If you're unsure about your technique, ask your GP practice nurse for a refresher demonstration — many practices offer support for patients self-administering injections.
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