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
Mounjaro Injection Sites — Can You Use Your Arm?
Yes, the arm is one of three approved injection sites for Mounjaro (tirzepatide), a weekly weight loss injection prescribed in the UK. However, injecting into your arm (upper arm) requires assistance from another person — you cannot safely do this yourself. This guide covers all approved Mounjaro injection sites, how to rotate between them safely, and what you need to know before your first injection. Using our GLP-1 weight loss calculator can help you understand potential outcomes alongside this medication.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
The Three Approved Mounjaro Injection Sites
Mounjaro is licensed for injection into three areas of your body:
- Stomach (abdomen) — the most common choice, easiest to self-inject
- Upper thigh — easy to self-inject, good alternative to stomach
- Upper arm — approved but requires another person to administer the injection
Each site must be rotated weekly to avoid lipohypertrophy (thickened fatty tissue) and ensure consistent absorption. The Mounjaro KwikPen pre-filled device comes with clear guidance on technique. Your prescriber or pharmacist will provide injection training before you start this prescription-only weekly weight loss injection. The arm injection site is particularly useful if you have abdominal sensitivity or prefer to rotate between upper arm and thigh for convenience.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Injection Sites and Safety Considerations
Proper site rotation is essential for safe, effective use of Mounjaro. Rotate your injection site each week — if you inject into your stomach one week, choose your thigh or arm the next week. Within the same body area, move the injection at least 2.5 cm away from your previous injection. Never inject into an area that is bruised, swollen, or tender from a previous injection.
The arm requires a helper — your partner, family member, or healthcare professional must administer the injection into your upper arm. This adds a practical consideration to your routine. If you struggle with access to assistance every week, the stomach or thigh may be more sustainable choices. Your GP or prescriber can discuss which rotation pattern works best for your lifestyle and the weekly weight loss injection routine you'll be following.
- 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 arm myself?
No — the upper arm is not recommended for self-injection. You will need another person, such as a partner or family member, to administer the injection safely into your upper arm. Your stomach or thigh are the easiest sites for self-injection. Your prescriber will provide injection training before you start this prescription-only medication.
Why do I need to rotate injection sites each week?
Rotating sites prevents lipohypertrophy (thickened, hardened fatty tissue under the skin) and ensures consistent absorption of the medication. Repeated injections in the same spot can reduce absorption and effectiveness of the Mounjaro dose. Move at least 2.5 cm away from your previous injection within the same body area.
What happens if I inject Mounjaro into a bruised area?
You should avoid injecting into any area that is bruised, swollen, or painful from a previous injection. This can increase discomfort and may affect absorption. If you have persistent bruising or reaction at injection sites, contact your GP or prescriber. They can advise on alternative sites or whether to pause and resume when the area heals.
Is the stomach or thigh easier than the arm for weekly injections?
Yes — most people find the stomach or thigh easier because you can self-inject without assistance. The arm requires a helper present every week, which adds logistical complexity. However, the arm is useful if you want to rotate between three sites or prefer not to inject into your stomach. Discuss your preference with your prescriber.
How do I know if my Mounjaro injection site is correct?
Your prescriber or pharmacist will demonstrate correct injection technique during your initial training. The injection should be into the fatty tissue (subcutaneous), not into muscle or blood vessels. You'll receive a demonstration with the Mounjaro KwikPen device. If you're unsure, ask your pharmacist to review your technique before your next dose.
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.