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
If you've been prescribed Mounjaro (tirzepatide) in the UK, knowing where and how to inject is essential for safe, effective treatment. The good news is that Mounjaro injections are straightforward to self-administer using the pre-filled KwikPen device. This guide explains the approved injection sites, how to rotate them properly, and how the injection process compares to other GLP-1 medications like Wegovy. Always follow your prescriber's instructions and the information leaflet provided with your KwikPen.
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Calculate Your BMI Instantly
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 vs Wegovy: Injection Sites and Technique
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) use similar injection sites, but there are important differences between them. Mounjaro is a dual GIP and GLP-1 receptor agonist — activating two hormone pathways — while Wegovy is a GLP-1 agonist only, which may explain differences in weight loss outcomes seen in clinical trials (SURMOUNT for Mounjaro showed average 15–clinically studied weight loss, whilst STEP trials for Wegovy showed approximately 15% average weight loss). However, both require once-weekly injections and use the same injection sites.
- Stomach (abdomen): Most common site for both medications. Inject 5cm below the belly button or to the side of the abdomen, avoiding the area 2cm around the navel.
- Upper thigh: Front and outer area of the thigh. Easy to access when on the go.
- Upper arm: Back of the upper arm. Requires assistance from another person to administer safely.
The key difference is that your prescriber will assess which medication is suitable for you based on your individual health needs and eligibility. Both are prescription-only and require a clinical assessment before being prescribed.
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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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Rotating Sites and Injection Best Practice
Rotate your injection site each week to prevent skin irritation, thickening (lipoatrophy), or lumpiness at injection sites. If you inject on Monday in your stomach, inject on the following Monday in your thigh, then your arm the week after, then switch sides. Never inject into the same site twice in a row. Keep your KwikPen at room temperature (below 30°C) for up to 30 days after first use; unopened pens should be stored in the fridge at 2–8°C. Before injecting, ensure your skin is clean and dry, and pinch the skin to create a firm surface. The needle goes in at a 90-degree angle. A prescriber or nurse can show you the correct technique during your initial consultation.
- 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 in the same spot each week?
No. Rotating injection sites is essential to prevent skin irritation, thickening, or lumps forming under the skin. Use a different site each week — stomach, thigh, arm, or alternate sides. Never inject into the same area twice in a row. Your prescriber or nurse will show you a rotation pattern.
Is Mounjaro injection more painful than Wegovy?
Pain sensation varies between individuals. Both use thin needles and most people report minimal discomfort. If you experience pain, check that your technique is correct, ensure the injection site is clean and dry, and avoid injecting into areas of irritated or hardened skin. A nurse can review your technique.
What's the difference between Mounjaro and Wegovy injections?
Both use pre-filled pens and similar injection sites (stomach, thigh, arm). Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, whilst Wegovy (semaglutide) is GLP-1 only. Mounjaro is once-weekly, Wegovy is once-weekly. Clinical trial data (SURMOUNT vs STEP) showed slightly higher average weight loss with Mounjaro, but individual results vary significantly.
How deep should the Mounjaro needle go?
The KwikPen needle is pre-set to inject subcutaneously (under the skin) at the correct depth. You don't control needle depth — simply hold the pen firmly against your skin at a 90-degree angle and press the injection button. The needle automatically delivers the full dose into the fatty tissue beneath the skin.
Can I use the same KwikPen for multiple injections?
Each KwikPen contains 4 weekly doses (a 4-week supply). After using all 4 doses, dispose of the pen safely in a sharps bin. Do not attempt to refill or reuse the pen. Store unused pens in the fridge; once opened, use within 30 days. Your prescriber will issue new pens as needed.
Ready to take control of your weight loss journey?
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