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
Can You Inject Mounjaro In The Arm?
Yes, you can inject Mounjaro in the upper arm, though the stomach and thigh are more commonly used first. Mounjaro (tirzepatide) is a prescription-only, once-weekly GLP-1 and GIP receptor agonist approved in the UK for weight management. Each injection site works equally well, and rotating sites each week helps prevent skin irritation and bruising. Your GP or prescriber will guide you on the best injection technique for your body and comfort level.
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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.
Approved Mounjaro Injection Sites
Mounjaro can be injected into three main areas:
- Stomach (abdomen) — most common starting site, typically around the navel area but not directly on it
- Upper thigh — the outer thigh, about a hand's width above the knee and below the hip
- Upper arm — the back or side of the upper arm, roughly halfway between shoulder and elbow; this site often requires help from another person
Rotating between sites each week is important to reduce the risk of lipohypertrophy (thickened skin tissue) and ensure consistent absorption. This rotation also minimises injection site reactions like redness, bruising, and discomfort. Always use a fresh needle for each injection and allow the skin to dry after cleaning.
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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.
Injecting Mounjaro In The Upper Arm — Practical Steps
If you choose the upper arm site, pinch a fold of skin gently and insert the needle at a 90-degree angle into the fatty tissue. The KwikPen device makes this easier than traditional syringes. Many people find the arm site more convenient than the stomach because it's easier to hide and rotate, though you may need assistance to reach the correct spot. Never inject into areas where the skin is tender, bruised, or scarred. Store your Mounjaro KwikPen in the fridge (2–8°C unopened), but it can be left at room temperature below 30°C for up to 30 days after first use.
- 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.
Is injecting Mounjaro in the arm as effective as the stomach?
Yes. All three approved injection sites (stomach, thigh, and upper arm) are equally effective. The medication is absorbed the same way regardless of location. Choose whichever site is most comfortable and convenient for you, and rotate weekly to avoid skin irritation.
Do I need help injecting Mounjaro in the arm?
Not always, but many people find it helpful. The back or side of the upper arm can be difficult to reach alone. If you can reach the site comfortably and see it in a mirror, self-injection is possible. If not, ask a partner, family member, or healthcare professional to help.
What should I do if I get bruising or redness at the arm injection site?
Some mild redness and bruising are normal and usually fade within a few days. Avoid injecting in the same spot, rotate sites each week, and apply a cold compress if needed. If bruising is severe, persistent, or painful, contact your GP or prescriber for advice.
Can I inject Mounjaro in the same arm twice?
You can use the same arm, but rotate the exact injection site each week within that arm. Injecting in the same spot repeatedly increases the risk of skin thickening and lipohypertrophy. Space injections at least 2 centimetres apart.
How do I know if I'm injecting Mounjaro correctly in the upper arm?
Pinch the skin to create a fold of fatty tissue, insert the needle at 90 degrees, and inject slowly. The KwikPen will click when the dose is released. You should feel minimal pain. If you're unsure of your technique, ask your GP, pharmacist, or nurse to observe your first few injections.
Ready to take control of your weight loss journey?
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