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 Monjaro — Safe Injection Sites
Monjaro (tirzepatide) is injected under the skin once a week, and choosing the right injection site is an important part of using this prescription weight loss medication safely. The KwikPen device is designed to make self-injection straightforward, but knowing where and how to inject will help you get the most from your treatment. This guide covers the approved injection sites, how to rotate between them, and practical tips for safe administration in the UK.
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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 Monjaro Injection Sites
Monjaro can be injected into three main areas of your body, all under the skin (subcutaneously). These sites have been chosen because they have sufficient fatty tissue and are easy to access:
- Stomach (abdomen) — The most commonly used site. Inject at least 5cm away from your belly button. This area typically has good fatty tissue and is convenient for self-injection.
- Upper thigh — An alternative site that works well for many people. You can inject on the front or outer side of the thigh.
- Upper arm (back of the arm) — Also approved, though many people find this site more difficult to access alone. You may want assistance from a partner or carer for this site.
All three sites are equally effective. The choice depends on your comfort, body composition, and what feels easiest for you to reach. Never inject into areas where the skin is tender, bruised, scarred, or hardened.
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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 Your Injection Sites
Rotating between sites is important to protect your skin and help with absorption of the tirzepatide. Rotate to a different site with each weekly injection — for example, stomach one week, thigh the next week, arm the third week, then back to stomach. Within each site area, choose a slightly different spot each time (at least 2.5cm away from the previous injection).
Rotating helps prevent lipodystrophy (changes to fatty tissue under the skin) and injection site reactions like redness or bruising. If you notice any persistent redness, swelling, warmth, or pain at an injection site that doesn't resolve within a few days, contact your GP or the person who prescribed your Monjaro.
- 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 Monjaro into the same spot every week?
No. You should rotate between different injection sites and different spots within each site area. Injecting in the same spot repeatedly can cause skin problems like lumps or thickened tissue (lipodystrophy). Rotating helps ensure good absorption and protects your skin health.
Which Monjaro injection site is best for beginners?
The stomach (abdomen) is usually easiest for beginners because it's easy to see and reach, and most people have sufficient fatty tissue there. Start here if you're new to injecting, then try rotating to the thigh or arm as you gain confidence. Always avoid the area around your belly button.
How deep should I inject Mounjaro into my skin?
The KwikPen is designed to inject at the correct depth automatically — you simply press the button on the device and hold it in place. The needle will penetrate about 5–8mm under the skin. You don't need to worry about measuring depth yourself; the pen does this for you.
What should I do if I accidentally inject into a scar or bruised area?
Avoid injecting into areas with scars, bruises, or hardened skin. If you accidentally inject into a problematic area, note it and choose a different site for your next injection. If you develop an unusual reaction (severe pain, large swelling, or infection signs), contact your GP.
Can I inject Monjaro through clothing?
No. Always inject directly into bare skin after cleaning the area with an alcohol wipe. Remove any clothing covering the injection site, let the skin dry completely after cleaning, and then inject. Never inject through fabric.
Ready to take control of your weight loss journey?
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