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 In Your Arm
If you've been prescribed Mounjaro (tirzepatide) in the UK, the upper arm is one of three approved injection sites. This guide explains exactly how to inject Mounjaro safely in your arm, including rotation techniques, needle insertion, and what to do if you experience injection site reactions. Always follow your prescriber's instructions and the patient information leaflet included with your KwikPen device.
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.
Mounjaro Arm Injection Sites And Technique
Mounjaro comes as a pre-filled KwikPen device and can be injected into three sites: the stomach (abdomen), upper thigh, or upper arm. For arm injections specifically, you should inject into the outer upper arm, roughly midway between shoulder and elbow. This area has sufficient subcutaneous fat for the medication to work effectively. The upper arm site requires assistance — you cannot easily inject yourself here, so you'll need help from a partner, family member, or healthcare professional. Before injecting, ensure the area is clean and dry. Pinch the skin gently, insert the needle at a 90-degree angle, and hold the KwikPen steady during the injection. The needle remains in place for three seconds after you press the injection button to ensure the full dose is delivered.
- Location: Outer upper arm, between shoulder and elbow
- Requires assistance from another person
- Skin should be clean, dry, and unpinched after injection
- Needle angle: perpendicular (90 degrees) to the skin
- Hold in place for 3 seconds after pressing the button
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 Your Injection Sites
Site rotation is important to prevent lipodystrophy (thickened or indented skin) at injection sites. If you're using the arm as one of your injection locations, rotate to a different site each week. For example, inject in your arm in week one, stomach in week two, thigh in week three, then back to the arm in week four. This spreads the injections across your body and allows each site time to recover. Keep a simple record of which site you used each week — many people use a calendar or the free progress tracker on MyWeightCheck to log their injections. Never inject into the same exact spot twice in a row. If you notice redness, bruising, warmth, or tenderness at a previous injection site, choose a different site that week.
- 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 own arm without help?
No. The upper arm site requires assistance from another person because the angle and depth needed for subcutaneous injection are difficult to achieve on yourself. You can self-inject into your stomach or thigh, but arm injections need help from a partner or healthcare professional.
What should I do if my arm injection site is bruised or swollen?
Mild bruising and swelling are common and usually resolve within a few days. Apply a cold compress for the first 10 minutes after injection if desired. Avoid the same spot for your next injection — rotate to a different site. If swelling, redness, or warmth persists beyond a few days or worsens, contact your GP or the prescriber who issued your Mounjaro prescription.
Is it better to inject Mounjaro in the arm, stomach, or thigh?
All three sites are effective. The arm requires assistance, making it less convenient for self-injection. Stomach and thigh injections are easier to do yourself. Many people rotate between all three sites weekly to minimise tissue irritation. Choose whichever site feels most comfortable and practical for your routine.
How deep should the needle go when injecting into the arm?
Mounjaro is a subcutaneous injection, meaning it goes into the fatty tissue beneath the skin, not into muscle. The KwikPen is designed to deliver the correct depth automatically — insert the needle at 90 degrees until it's fully inserted, then press the button. You don't need to push harder or adjust the depth manually.
What is tirzepatide and how does Mounjaro differ from Wegovy?
Tirzepatide is the active ingredient in Mounjaro. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it works on two hormone pathways. Wegovy (semaglutide) activates only the GLP-1 pathway. Both are prescription-only weight management injections used in the UK, but Mounjaro typically requires higher doses (up to 15 mg weekly) compared to Wegovy (up to 2.4 mg weekly). Clinical trials showed Mounjaro may produce slightly greater average weight loss, though individual results vary considerably. ```
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.