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 Do You Inject Mounjaro?
Mounjaro (tirzepatide) is injected under the skin once weekly using a pre-filled KwikPen device. You can choose from three injection sites: the stomach (abdomen), upper thigh, or upper arm. Rotating your injection site each week helps prevent irritation and bruising at the same spot. While the stomach is the most common choice for self-injection, all three areas are equally effective. This guide explains exactly where to inject Mounjaro, how to rotate sites safely, and how the injection process compares to Wegovy (semaglutide).
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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.
Mounjaro Injection Sites and Rotation Technique
Mounjaro can be injected into three areas of the body, and rotating injection sites is essential to prevent skin irritation, bruising, and lipodystrophy (changes in fat tissue at injection sites).
- Stomach (abdomen): Most common and easiest for self-injection. Inject into the fatty tissue 5cm around your belly button — avoid injecting directly into scars or areas with bruising.
- Upper thigh: Equally effective. Pinch the skin to create a fold, then inject into the fatty layer. Front or outer thigh works best.
- Upper arm: Most difficult for self-injection. Requires assistance from another person or a healthcare professional. Inject into the back of the upper arm.
Each week, inject into a different site from your previous injection. If rotating between arms and legs, wait at least two weeks before using the same arm or leg again. Always leave at least one finger-width distance between injection sites.
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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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Mounjaro vs Wegovy: Injection Site Differences
Both Mounjaro and Wegovy use identical injection site guidance — stomach, thigh, or upper arm. Both are once-weekly injectable medications delivered via pre-filled pens (Mounjaro uses a KwikPen, Wegovy uses an auto-injection pen). The injection technique is essentially the same for both medications. The key difference is the active ingredient: Mounjaro contains tirzepatide (a dual GIP and GLP-1 receptor agonist), whilst Wegovy contains semaglutide (a GLP-1 receptor agonist only). Clinical trial data shows Mounjaro produced average weight loss of approximately 20.9% at the maximum dose (15mg), whilst Wegovy produced approximately 14.9% in trials — but individual results vary significantly. Both medications are prescription-only and require a clinical assessment before being prescribed.
- 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 twice?
No. Always rotate injection sites to prevent skin irritation, bruising, and lipodystrophy. Use a different area each week. If you rotate between the stomach, thigh, and arm, wait at least two weeks before returning to the same area.
Is the stomach the best place to inject Mounjaro?
The stomach (abdomen) is the most common injection site for self-administration because it's easiest to access. However, all three sites (stomach, thigh, upper arm) are equally effective. Choose based on comfort and convenience, but rotate weekly.
Can someone else inject my Mounjaro for me?
Yes. If you struggle with self-injection, a partner, carer, or healthcare professional can inject you. The upper arm is typically easier for another person to inject. Your prescriber or pharmacist can teach you or your carer the correct technique.
How deep do you inject Mounjaro?
Mounjaro is injected into the subcutaneous fat layer (under the skin, above the muscle). The KwikPen is designed to deliver the correct injection depth automatically — you simply push it against your skin and press the button. No a UK pharmacy depth control is needed.
What should I do if I get bruising or redness at the injection site?
Minor bruising or redness is common and usually resolves within a few days. Rotate to a different site next week. Seek medical advice if you notice severe pain, swelling, infection signs (warmth, pus), or if bruising persists beyond a week.
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