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 Mounjaro: Safe Injection Sites and Technique
Mounjaro (tirzepatide) is a once-weekly prescription injection that you administer yourself using a pre-filled KwikPen device. The medication can be injected into three main body areas: your stomach (abdomen), upper thigh, or upper arm. Proper injection site rotation is important to help prevent skin irritation and maintain consistent medication absorption. This guide explains exactly where to inject Mounjaro, how to rotate sites correctly, and how the injection process compares to Wegovy.
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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 vs Wegovy: Injection Site Comparison
Both Mounjaro and Wegovy are once-weekly prescription injections that you inject yourself at home using pre-filled pen devices. While the injection technique and site rotation principles are identical, the medications differ in mechanism and dosing.
- Mounjaro (tirzepatide): Dual GIP + GLP-1 receptor agonist. Doses range from 2.5 mg to 15 mg once weekly. Uses the KwikPen device. Clinical trials (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight at maximum dose alongside reduced-calorie diet and physical activity.
- Wegovy (semaglutide): GLP-1 receptor agonist only. Doses range from 0.25 mg to 2.4 mg once weekly. Uses a pre-filled auto-injection pen. Clinical trials (STEP-1) showed average weight loss of approximately 15% of body weight at maximum dose with lifestyle changes.
- Injection sites are identical for both: stomach, upper thigh, or upper arm. Rotation between sites is equally important for both medications to prevent tissue damage and lipohypertrophy (fatty lumps under the skin).
Both Mounjaro and Wegovy are prescription-only medicines requiring a clinical assessment before being prescribed. Individual suitability depends on your personal health profile, which a UK-licensed prescriber will assess.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Mounjaro Injection Site Rotation: Best Practice
Rotate your injection site every week to prevent skin irritation, bruising, and lipohypertrophy (abnormal fat accumulation). This means if you inject your stomach one week, choose your thigh the next week, then your upper arm, then back to stomach. Within each body area, rotate specific injection points so you do not inject in the exact same spot twice in a row.
Always inject at least one finger width away from any previous injection site, scars, or areas of redness or bruising. Allow injection sites to fully heal before reusing the same area. If you notice persistent redness, lumps, numbness, or pain at injection sites, inform your GP or prescriber. Use our free GLP-1 weight loss calculator to track your progress alongside your Mounjaro treatment, and log your injection dates in our progress tracker to help you stay organised with your weekly doses.
- 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.
What are the three Mounjaro injection sites?
The three approved injection sites are your stomach (abdomen), upper thigh, and upper arm. You can inject anywhere on your stomach between your ribs and hip. Your thigh injection area extends from your knee to your hip. Upper arm injections work best on the outer area. Always rotate between different body areas each week.
How do I rotate Mounjaro injection sites correctly?
Rotate between the three body areas weekly: stomach one week, thigh the next week, upper arm the following week, then back to stomach. Within each area, choose a slightly different spot each time — at least one finger width away from your previous injection. This prevents skin damage and lipohypertrophy.
Can I use the same injection site twice in a row?
No. Never inject in the same spot twice consecutively. You should wait at least one week before reusing the exact same injection point. This helps prevent bruising, skin thickening, and fatty deposits. Proper rotation ensures even medication absorption across all injection sites.
Is Mounjaro injection technique different from Wegovy injection technique?
No. Both medications use identical injection site rotation principles and the same three body areas (stomach, thigh, upper arm). The main differences are the device type (Mounjaro uses KwikPen, Wegovy uses auto-injection pen) and the medication mechanisms (Mounjaro is dual GIP+GLP-1, Wegovy is GLP-1 only). Both require weekly rotation.
What should I do if my Mounjaro injection site becomes red or swollen?
If you experience redness, swelling, bruising, numbness, or pain at an injection site, switch to a different body area immediately and allow the affected site to heal. Avoid injecting in that area for at least one week. If redness persists beyond a few days or spreads, contact your GP or prescriber as this could indicate an infection or allergic reaction.
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