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 Sites and Technique
Monjaro (tirzepatide) is a once-weekly weight loss injection that you can administer yourself at home using the pre-filled KwikPen device. Knowing the correct injection sites and how to rotate them properly is essential for safe, comfortable use and to reduce side effects like injection site reactions. This guide explains exactly where you can inject Monjaro, how to rotate sites, and what to expect during the injection process based on UK prescribing guidance and manufacturer information.
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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.
Monjaro Approved Injection Sites
Monjaro can be injected into three main areas of your body. The stomach (abdomen) is the most commonly used site — inject into the soft tissue at least 2 inches (5cm) away from your belly button. The upper thigh is the second option — use the outer area of either thigh. The upper arm can also be used but typically requires assistance from another person to inject safely. Each site should be rotated weekly to prevent lipohypertrophy (thickened skin) and ensure consistent medication absorption. Never inject into areas where the skin is tender, bruised, red, hard, or thick. Always leave at least 1 inch (2.5cm) between injection sites if using the same area twice in a month. The pre-filled KwikPen makes self-injection straightforward — the needle automatically retracts after use.
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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 Site Rotation and Best Practices
Rotating your injection site each week helps prevent local tissue damage and ensures your body absorbs the tirzepatide consistently. Many people use a simple rotation pattern: Week 1 stomach, Week 2 left thigh, Week 3 right thigh, Week 4 right arm (with assistance), then repeat. Keep the skin clean and dry before injecting — you don't need to use alcohol swabs for every injection, but ensure the area is clean. Allow the pen to reach room temperature if it's been refrigerated. Always check your skin for signs of injection site reactions like redness, swelling, or itching; these usually fade within a few hours but report persistent reactions to your prescriber. Using our progress tracker can help you log which sites you've used and identify any patterns in side effects or comfort levels.
- 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 in the same spot twice?
It's best to avoid the exact same spot. Always leave at least 1 inch (2.5cm) between injection sites if reusing an area within a month. Rotating sites prevents lipohypertrophy (thickened or hardened skin patches) and ensures consistent absorption of the tirzepatide dose.
Does Monjaro injection hurt?
Most people report minimal pain — the needle is very fine (31-gauge). You may feel a slight pinch, but the injection usually takes only a few seconds. If you experience significant pain, stop and contact your prescriber. Injection site reactions like mild redness or bruising are common and typically resolve within hours.
Can someone else inject me with Monjaro?
Yes. If you prefer, a family member or healthcare professional can administer your Monjaro injection, particularly if using the upper arm site. The person injecting should follow the same guidance: clean the site, allow the pen to warm to room temperature, and rotate injection areas weekly.
What if I inject Monjaro incorrectly?
If you suspect you've injected incorrectly — for example, the needle didn't fully deploy or you withdrew early — contact your prescriber or pharmacist for advice. Do not inject a second dose. They can advise whether to reschedule your dose or proceed as normal depending on the circumstances.
How do I store Monjaro safely at home?
Unopened pens must be refrigerated at 2–8°C in the original carton. After first use, you can store the pen at room temperature (below 30°C) for up to 30 days. Never freeze Monjaro — if frozen, discard it. Keep away from heat and direct sunlight. Always check the expiry date on your pen.
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