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
Free tracker included with every download this week
Where Should I Inject Mounjaro?
Mounjaro (tirzepatide) comes as a pre-filled KwikPen device and can be injected into three main areas of your body. The stomach (abdomen) is the most common injection site, though your thighs and upper arms are also suitable locations. This guide explains the approved injection sites, how to rotate them safely, and what to expect during your weekly injection. 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 areas of your body:
- Stomach (abdomen) — the most commonly used site. Inject into the fatty tissue on your lower stomach, at least 5cm away from your belly button. This is often the easiest site to access yourself.
- Upper thigh (quadriceps) — inject into the outer or front area of your thigh. You can do this yourself, and it's a good alternative if your stomach feels sore.
- Upper arm (back of triceps) — the back of your upper arm, into the fatty tissue. This site typically requires assistance from another person because it's difficult to reach safely yourself.
Your prescriber or nurse will demonstrate the correct injection technique during your first dose. Always use the KwikPen exactly as shown in the training video or patient guidance.
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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 Your Injection Sites
Rotating injection sites is essential to prevent skin damage and lipodystrophy (changes in fat tissue). Inject into a different location each week — for example, Week 1: left stomach, Week 2: right stomach, Week 3: left thigh, Week 4: right thigh, then cycle through again. Always choose an area at least 2–3cm away from your previous injection. Never inject into areas where the skin is bruised, tender, scarred, or hardened. If you notice any persistent redness, lumps, or indentation, inform your GP. Before injecting, check that your skin is clean and dry, and let the KwikPen sit at room temperature for 15 minutes if taken from the fridge.
- 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 myself?
Yes. The stomach and thigh are designed for self-injection. The upper arm site typically requires help from another person. Your healthcare provider or nurse will train you on the correct technique during your first dose. Always use a new needle for each injection and dispose of used needles safely in a sharps bin.
What happens if I inject Mounjaro into the wrong spot?
Injecting slightly off the intended area usually causes no serious harm, but accuracy improves absorption and reduces local skin irritation. If you're unsure, contact your GP or the pharmacist who dispensed your prescription. Never re-inject if you've already given yourself a dose, even if you think you missed the intended site.
Should I inject Mounjaro into muscle or fatty tissue?
Always inject into subcutaneous fatty tissue, not muscle. Mounjaro is a subcutaneous injection — it goes into the layer of fat under your skin, not into muscle. Injecting into muscle can cause pain and may affect how the medication works. Your prescriber will confirm the correct depth and technique.
How do I know if my injection sites are getting damaged?
Watch for persistent redness, lumps, indentation, or hardened areas of skin. These signs suggest lipodystrophy. If you notice any of these, inform your GP and rotate your injection sites more frequently. Avoid injecting into affected areas until the skin has recovered.
Can I use the same injection site twice in one week?
No. Rotate to a different site each week to allow the skin to recover. Reusing the same site repeatedly increases the risk of skin damage, infection, and poor medication absorption. A typical rotation pattern uses four or more different sites across your body.
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.