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 in Your Arm
Mounjaro (tirzepatide) can be injected into the upper arm, though the abdomen and thigh are more commonly used. This guide explains safe arm injection technique, how it compares to Wegovy injection sites, and why proper injection site rotation matters for your treatment. Whether you're starting tirzepatide or considering it, understanding injection mechanics helps you feel confident with your once-weekly dose.
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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 Arm Injection Sites and Technique
The upper arm is an approved injection site for Mounjaro, though many UK patients prefer the abdomen or thigh for easier self-injection. If you choose to inject into your arm, the ideal location is the outer upper arm area — between the shoulder and elbow on the back or side of the arm. The skin here has sufficient subcutaneous tissue for a subcutaneous injection.
When injecting into your arm yourself, you may find it awkward to reach; many patients ask a partner, family member, or nurse to administer the injection at this site instead. The Mounjaro KwikPen is pre-filled and simple to use — you hold the pen at a 90-degree angle to the skin, press the button, and hold for 3 seconds. Each dose is a once-weekly injection, and you should rotate injection sites each week to avoid lipohypertrophy (thickened skin from repeated injections at the same spot). This rotation across abdomen, thighs, and upper arms helps maintain healthy tissue and supports consistent absorption of the tirzepatide dose.
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Mounjaro vs Wegovy: Injection Site Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) use the same three approved injection sites: upper arm, abdomen, and thigh. The main difference is mechanism — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy) showed comparable weight loss results, though Mounjaro achieved slightly higher average weight loss in trials (15–21% body weight vs 14.9% for Wegovy). Site rotation guidance is identical for both: change your injection location weekly to maintain skin health. Your GP or prescriber will assess which medication suits your individual health profile — both require prescription and clinical assessment in the UK.
- 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 my upper arm myself?
Yes, but it can be difficult to reach. Many UK patients ask a partner, family member, or nurse to administer the injection at the upper arm site. The abdomen and thigh are easier for self-injection. Your prescriber or diabetes nurse can demonstrate the technique during your first dose.
What happens if I inject Mounjaro in the same arm spot every week?
Repeated injections at the same site can cause lipohypertrophy — thickened or hardened skin — which may affect absorption of the tirzepatide. The NHS recommends rotating injection sites weekly across your abdomen, thighs, and upper arms to prevent this and ensure consistent dosing.
Is Wegovy injection technique different from Mounjaro?
No, both use identical approved sites: upper arm, abdomen, and thigh. Both are once-weekly injections using pre-filled pens. The difference is in the active ingredient — tirzepatide (Mounjaro) vs semaglutide (Wegovy) — not in injection method.
Should I warm my arm before injecting Mounjaro?
There is no requirement to warm the injection site. Some patients find room-temperature skin easier to inject into than cold skin, but this is personal preference. Allow the pre-filled pen to sit at room temperature for 15 minutes before use if it has been refrigerated.
How do I know I've injected Mounjaro into the right depth?
The Mounjaro KwikPen is designed for subcutaneous injection — just under the skin. You hold the pen at 90 degrees to the skin and press the button. You'll hear a click and feel it inject. The needle length is fixed and correct for subcutaneous delivery. If you're unsure, ask your GP or nurse to observe your first injection.
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