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 Can You Inject Monjaro?
Monjaro (tirzepatide) is injected into the fatty tissue just under your skin, and there are three approved injection sites to rotate between. The most common site is the stomach (abdomen), but your upper thigh and upper arm are also suitable locations. Rotating your injection site each week helps prevent skin irritation and ensures even distribution of the medication. Your prescriber will show you how to inject safely before you start, and the pre-filled KwikPen device is designed for easy self-administration at home.
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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 comes as a pre-filled KwikPen injection device and can be administered into three specific areas of the body:
- Stomach (abdomen) — the most commonly used site. Inject into the fatty tissue at least 5cm away from your belly button. This is often the easiest site for self-injection.
- Upper thigh (front or outer surface) — a convenient alternative site. Ensure you pinch the skin to create a fold before injecting.
- Upper arm (back of the arm) — the triceps area. This site typically requires assistance from another person, as it's difficult to self-inject here safely.
Each week, rotate to a different site to minimise tissue damage and injection site reactions. Never inject into the same spot twice in a row. This GIP receptor agonist combined with GLP-1 receptor agonist mechanism works best when given as a subcutaneous injection into fatty tissue, not into muscle or veins.
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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.
Safe Injection Technique and Site Care
Before each injection, inspect the KwikPen to ensure the medication is clear and colourless. Allow the pen to reach room temperature if stored in the fridge (do not apply heat). Clean the injection site with an alcohol wipe and allow it to dry. Pinch the skin gently to create a fold, insert the needle at a 90-degree angle, and hold the pen steady during injection. After injecting, keep the pen in place for 6 seconds. Dispose of used needles in a sharps bin — never reuse needles. If you experience redness, bruising, or pain at injection sites that persists beyond a few days, contact your GP or prescriber. Store unopened pens at 2–8°C in the original carton. Once opened, a pen can be kept at room temperature (below 30°C) for up to 30 days.
- 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 my arm myself?
The upper arm site (triceps area) is approved for Monjaro injection, but it's very difficult to inject yourself safely into this location. Most people require assistance from another person. The stomach and thigh are easier for self-injection and are recommended for most users.
What should I do if I get a bruise or redness after injecting Monjaro?
Minor redness, bruising, or mild pain at the injection site is common and usually settles within a few days. Always rotate your injection site weekly to minimise reactions. If redness persists beyond 3 days, spreads significantly, or becomes painful, contact your GP or prescriber for advice.
How do I know if I'm injecting Monjaro into the right location?
You should inject into fatty tissue (subcutaneous fat) just under the skin, not into muscle. If you're uncertain about technique, ask your prescriber to demonstrate before your first injection. Pinching the skin to create a fold helps ensure you're injecting into fatty tissue correctly.
Can I change my injection site during the same week?
No — Monjaro should be injected once per week into a different site each week. Stick to your chosen site for that week, then rotate to a different site the following week. This schedule allows tissue to recover and reduces injection site reactions.
What if I accidentally inject Monjaro into muscle instead of fatty tissue?
Accidentally injecting into muscle is unlikely to cause serious harm, but it may affect how quickly the medication is absorbed. If you're concerned you injected incorrectly, contact your prescriber or GP. They can advise whether you need any action. Always use the pinch technique to ensure you're in fatty tissue.
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.