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 to Inject Mounjaro — Safe Injection Sites
Mounjaro (tirzepatide) is injected once weekly into specific body areas to help with weight management in UK adults. The injection sites are straightforward to use with the pre-filled KwikPen device, and rotating between different areas helps reduce skin irritation and ensures consistent absorption. Understanding the correct injection points is an important part of using this prescription-only medication safely and effectively. Your prescriber will show you how to inject, but it's helpful to know the recommended sites beforehand.
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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 four main areas of your body, and rotating between sites each week is important to prevent skin irritation and lipohypertrophy (thickened skin patches).
- Stomach (abdomen) — the most commonly used site; inject at least 2 inches (5 cm) away from your belly button, and avoid areas with bruises, scars, or tender skin
- Upper thigh — a practical alternative; inject into the front or outer area of the thigh
- Upper arm — the back of the upper arm (triceps area); this site requires assistance from another person, as you cannot easily reach it yourself
- Buttocks — some guidance includes the buttocks as a fourth option, though less commonly discussed in UK practice
The key principle is rotation: if you inject into your stomach this week, choose your thigh next week, then upper arm the following week. This helps your skin stay healthy and the medication absorb reliably.
Clinically BackedWeight Loss
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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.
Important Injection Site Safety Tips
Before each injection, inspect your skin at the chosen site — avoid any areas with active bruising, swelling, hardness, redness, or pain. Never inject directly into scar tissue or areas affected by lipodystrophy. The injection should go into fatty tissue just below the skin (subcutaneous injection), not into muscle or deeper tissues.
Allow the KwikPen to sit at room temperature for 15–30 minutes before injecting if it has been refrigerated — this makes the injection more comfortable. Pinch the skin gently to create a fold, insert the needle at a 90-degree angle, and hold for the full duration specified in your instructions. After injecting, dispose of the used pen safely in a sharps bin or container. If you have questions about your specific injection technique, ask your GP or the prescriber who dispensed your medication.
- 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 into the same spot every week?
No. Rotating injection sites every week is essential to prevent skin complications like lipohypertrophy (thickened patches) and ensure consistent absorption of the medication. Use a different site each week — for example, stomach one week, thigh the next, then upper arm.
Does it hurt to inject Mounjaro?
Most people report minimal discomfort with the Mounjaro KwikPen. The needle is very thin. Allowing the pen to reach room temperature before injection, and relaxing the injected area, can help reduce any discomfort. If you experience pain at injection sites, discuss this with your prescriber.
What should I do if I notice bruising or swelling at an injection site?
Minor bruising and slight swelling are common and usually resolve within a few days. Avoid injecting into that area for at least one week. If swelling is severe, warm to the touch, or doesn't improve, contact your GP to rule out infection or other complications.
Can someone else inject Mounjaro for me if I'm nervous?
Yes. Another person can administer your Mounjaro injection, particularly if you're using the upper arm site (which is difficult to reach yourself). Make sure they're trained on the correct technique by your prescriber or a healthcare professional before attempting the injection.
Is rotating injection sites connected to weight loss effectiveness?
Rotating sites doesn't affect how much weight the medication helps you lose, but it does ensure reliable absorption and prevents painful skin complications. Consistent, comfortable injections support long-term adherence to your weight management plan.
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.