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 Do You Inject Mounjaro? UK Injection Site Guide
Mounjaro (tirzepatide) is injected under the skin once a week, and you have several safe injection site options. In the UK, the most common injection sites are the stomach (abdomen), upper thigh, and upper arm — you'll rotate between sites each week to reduce irritation. Your prescriber will show you how to inject properly when you start treatment, and the Mounjaro KwikPen is designed to be straightforward to use at home. Understanding the correct injection technique helps ensure the medicine works effectively and minimises discomfort.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 Explained
Mounjaro comes as a pre-filled KwikPen device that you use to inject the tirzepatide under the skin (subcutaneously) once weekly. There are three main injection sites approved for Mounjaro:
- Stomach (abdomen) — the most commonly used site. Inject into the area around your belly button, avoiding the area directly above your hip bones and at least 2.5cm from your belly button.
- Upper thigh — inject into the front and outer area of your thigh. This is a convenient option if you prefer to inject yourself without assistance.
- Upper arm — inject into the back of your upper arm. This site typically requires help from another person to inject safely and accurately.
Site rotation is essential: rotate your injection site each week to prevent lipohypertrophy (thickened fatty tissue) and ensure consistent medicine absorption. Never inject into the same spot two weeks in a row. If you notice redness, bruising, or soreness at an injection site, choose a different location for your next dose.
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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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NHS Mounjaro Prescription and Injection Guidance
If you receive Mounjaro through NHS specialist weight management services, your prescriber or pharmacist will provide detailed instruction on injection technique before you start treatment. The NHS recommends checking each injection site before injecting — the skin should be clean and dry, and you should avoid injecting into areas with scars, bruises, or irritation. Your healthcare team will also advise you on safe needle disposal using a sharps bin. When using the GLP-1 weight loss calculator on this page, you can estimate potential results with Mounjaro at different doses; however, injection technique must always be confirmed with your prescriber or nurse. Store your KwikPen in the refrigerator until use, and keep a record of your injection sites to ensure proper rotation.
- 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 myself at home in the UK?
Yes, Mounjaro is designed for self-injection at home once you've been trained. Your prescriber or nurse will demonstrate the correct technique. Most people use the stomach or thigh for self-injection. Always follow the written instructions provided with your KwikPen, and contact your GP or pharmacy if you're unsure about your technique.
What happens if I inject Mounjaro into the same spot twice?
Injecting repeatedly into the same site can cause lipohypertrophy (thickened skin) and affect medicine absorption. This is why rotating injection sites each week is important. NHS guidance recommends choosing a different site each dose. If you've accidentally used the same area, inform your prescriber at your next appointment.
Is the upper arm injection site safe for Mounjaro?
Yes, the upper arm is a safe injection site approved for Mounjaro in the UK. However, injecting into the back of your arm typically requires assistance from another person to ensure accuracy and safety. The stomach and thigh are easier for self-injection.
How do I know if my Mounjaro injection site is infected?
Signs of infection include increased redness, warmth, swelling, pus, or increasing pain at the injection site lasting more than a few days. If you notice these signs, contact your GP or call NHS 111. Most injection site reactions (mild redness or bruising) resolve within a few days with no treatment needed.
Can I use the same KwikPen needle for multiple injections?
No. Each Mounjaro KwikPen comes with a single needle attached, and you must use a new needle for each injection. Never reuse needles — this increases infection risk and reduces medicine effectiveness. Dispose of used needles safely in a sharps bin provided by your pharmacy.
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