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 — UK Injection Site Guide
Mounjaro (tirzepatide) is a once-weekly weight loss injection that you administer yourself using a pre-filled KwikPen device. Knowing the correct injection sites is essential for effective delivery and reducing discomfort. The NHS and MHRA-approved guidance identifies three safe areas on your body where you can inject Mounjaro. Proper injection technique and site rotation help prevent skin irritation and ensure consistent absorption of this prescription-only weekly weight loss injection.
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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 — Where to Inject
Mounjaro can be injected into three areas of your body, as approved by the MHRA and detailed in the product information:
- Stomach (abdomen): Most commonly used site. Inject into the fatty tissue around your belly, avoiding the area 5cm (2 inches) around your navel. This is the easiest site for self-injection and offers reliable absorption.
- Upper thigh: The front and outer area of your thigh (quadriceps region) is an excellent alternative. Pinch the skin to create a fold before injecting into the fatty tissue underneath.
- Upper arm: The back of your upper arm (triceps area) can be used, but typically requires assistance from another person to inject safely and accurately.
Each site should be rotated weekly to prevent lipodystrophy (skin thickening or indentation) and ensure consistent medication absorption. Always choose a different site from your previous injection, and avoid injecting into areas where the skin is bruised, hardened, scarred, or tender.
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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, Rotation, and Storage Tips
Before injecting: Check that your Mounjaro KwikPen has been stored correctly (in the fridge at 2–8°C if unopened, or at room temperature below 30°C for up to 30 days after first use). Allow the pen to warm to room temperature for 15–30 minutes if taken from the fridge. Wash your hands thoroughly and clean the injection site with an alcohol wipe.
Injection rotation: Use a different site each week to minimise irritation. Many people follow a pattern: stomach week 1, left thigh week 2, right thigh week 3, upper arm week 4, then restart. After injecting: Apply light pressure with a clean gauze pad for a few seconds if needed. Do not rub the area. If you experience persistent pain, redness, or swelling at injection sites, contact your GP or prescriber. Track your progress and weight loss alongside injection management using our free progress tracker to monitor how well your treatment plan is working.
- 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 muscle rather than fatty tissue?
No. Mounjaro must be injected into subcutaneous fatty tissue (fat under the skin), not muscle. Injecting into muscle can affect absorption and increase the risk of side effects. Always pinch the skin to create a fold and inject into the fatty tissue beneath, not deeper into muscle.
What should I do if I inject Mounjaro into the same site twice?
Occasionally using the same site twice is not dangerous, but it increases the risk of skin irritation, lipodystrophy, and reduced medication absorption. Rotate sites consistently to prevent these issues. If you've accidentally reused a site, simply ensure your next injection is in a different location. Contact your GP or prescriber if you develop persistent irritation.
Is it normal to experience bruising or redness at Mounjaro injection sites?
Minor redness or mild bruising is common and usually resolves within a few hours to days. Rotating injection sites, using proper technique, and avoiding rubbing the area helps minimise this. However, if you experience severe pain, significant swelling, warmth, or signs of infection (pus, increasing redness), contact your GP immediately.
Can someone else inject Mounjaro for me if I'm unable to self-inject?
Yes, another person can administer your Mounjaro injection if you're unable to self-inject. This is particularly useful for upper arm injections, which require good visibility and flexibility. Ensure they follow the same sterile technique and site rotation principles. Your prescriber or pharmacist can demonstrate the correct technique.
How do I know if my Mounjaro injection site is correct?
The correct site is an area of fatty tissue with good blood supply. The stomach, upper thigh, and upper arm are the approved sites. You should feel the needle enter the fatty tissue beneath the skin—not bone or deeper muscle. If you feel pain or resistance, stop and contact your pharmacist. Using the site-checking technique (pinching the skin) helps confirm adequate fatty tissue is available.
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