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?
Mounjaro (tirzepatide) is a once-weekly injection that you administer yourself using a pre-filled KwikPen. Choosing the right injection site and rotating between locations is important for comfort, effectiveness, and reducing side effects like injection site reactions. This guide explains the approved Mounjaro injection sites, how to rotate safely, and what to expect during administration. Always follow your prescriber's instructions and the guidance in your patient information leaflet.
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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.
Approved Mounjaro Injection Sites
Mounjaro can be injected into three main areas of your body, all of which are equally effective. The stomach (abdomen) is the most commonly used site — inject into the skin fold around your belly, avoiding the area directly around your navel. The upper thigh is another popular option; you can inject into the outer area of either thigh. The upper arm is also approved, though it typically requires assistance from another person to administer safely.
- Stomach: most common choice for self-injection
- Thigh: convenient alternative, easy to rotate between legs
- Upper arm: requires help, good for rotation variety
Each site works equally well. The key is choosing whichever feels most comfortable and accessible for you personally. Your prescriber or pharmacist can demonstrate the injection technique during your clinical assessment.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Rotating Injection Sites and Safety Tips
Rotating your injection site is crucial for comfort and reducing injection site reactions like redness, bruising, or pain. Change your injection location each week — for example, alternate between stomach one week, thigh the next, then arm. Never inject into the exact same spot repeatedly, as this can cause lipohypertrophy (thickened skin tissue).
- Inject at least 2.5 cm (1 inch) away from your previous injection site
- Avoid areas with bruising, scars, or tender skin
- Allow your skin to dry after cleaning with alcohol swabs before injecting
- Pinch the skin gently during injection for comfort
If you experience persistent injection site reactions, inform your GP or prescriber. Most reactions are mild and resolve within a few days with proper site 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 into the same spot every week?
No. You should rotate your injection site each week to prevent lipohypertrophy (thickened skin) and injection site reactions. Change between stomach, thigh, and upper arm locations, and always inject at least 2.5 cm away from your previous injection site. Proper rotation ensures comfort and maintains the effectiveness of the medication.
Is the stomach the best place to inject Mounjaro?
The stomach (abdomen) is the most commonly used site because it's easy to self-inject, but the thigh and upper arm are equally effective. Choose the site that feels most comfortable for you. What matters most is rotating between all three sites consistently to prevent skin irritation and maintain optimal absorption.
What should I do if I get a reaction at my injection site?
Mild redness, bruising, or pain at the injection site is common and usually resolves within a few days. Ensure you're rotating sites properly and allowing your skin to dry after cleaning. If reactions persist, become severe, or show signs of infection (increasing redness, warmth, pus), contact your GP or prescriber immediately.
Do I need help injecting Mounjaro into my upper arm?
Yes, the upper arm is the only site that typically requires assistance because of the angle and difficulty reaching the injection point yourself. If you prefer self-injection, stomach and thigh are better options. Your prescriber or pharmacist can demonstrate all injection techniques during your clinical assessment and training.
How long does a Mounjaro injection take?
A Mounjaro injection takes just a few seconds to administer — typically 5 to 10 seconds from needle insertion to withdrawal. The entire process, including preparing your pen and cleaning your skin, usually takes less than two minutes. Most users find it quick and straightforward once they've had proper training.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.