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 Is The Most Effective Place To Inject Mounjaro?
Mounjaro (tirzepatide) is injected once weekly, and the injection site you choose doesn't significantly affect how well the medication works — all approved sites are equally effective. However, proper technique and rotating between sites matters for comfort and skin health. This guide explains the best places to inject Mounjaro using your KwikPen, how to rotate sites correctly, and answers common questions about injection administration. Use our GLP-1 weight loss calculator to explore potential outcomes with Mounjaro alongside this practical guidance.
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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.
Approved Mounjaro Injection Sites
Mounjaro can be injected into three main areas of your body. All sites are equally effective — the choice comes down to personal preference and comfort. The stomach (abdomen) is the most commonly used site, as it's easy to access and self-inject. The upper thigh is the second most popular option and also easy to reach yourself. The upper arm can be used but typically requires assistance from another person to inject safely. Each site has a large area you can use, so you can rotate within and between sites each week. This rotation helps prevent skin irritation, bruising, and lipohypertrophy (thickening of tissue under the skin). Always check your patient information leaflet or ask your prescriber if you have questions about specific injection sites.
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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.
How To Rotate Mounjaro Injection Sites Correctly
Site rotation is important for comfort and skin health. The NHS guidance and manufacturer guidance recommend rotating your injection site each week — ideally moving to a different area (stomach, thigh, or arm) rather than injecting in exactly the same spot repeatedly. If you use your stomach one week, choose your thigh the next week, then your arm the following week, then return to your stomach but in a slightly different location. Within each area, aim to inject at least 2–3 cm (about one finger-width) away from your previous injection site. Avoid areas where you've recently injected (within the last 2 weeks ideally). Never inject into areas that are bruised, tender, scarred, or have hardened tissue. Proper rotation prevents complications and ensures consistent medication absorption.
- 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 one Mounjaro injection site more effective than others?
No. The stomach, upper thigh, and upper arm are all equally effective for administering Mounjaro. Absorption and effectiveness are consistent across all approved sites. Your choice should be based on comfort, ease of access, and which site you can rotate to effectively each week.
Can I inject Mounjaro in the same spot every week?
It's not recommended. Injecting in the same spot repeatedly can cause bruising, skin thickening, and tissue damage. The manufacturer and NHS guidance recommend rotating sites weekly — ideally alternating between stomach, thigh, and arm, or moving 2–3 cm away if using the same general area.
What should I do if my Mounjaro injection site is bruised?
Mild bruising is common and usually resolves within a few days. Avoid injecting near the bruised area for at least 2 weeks. Apply a cold compress immediately after injection to reduce bruising. If you experience severe bruising, pain, or signs of infection (warmth, redness, pus), contact your GP or prescriber.
Do I need to warm Mounjaro before injecting?
No. You can inject Mounjaro directly from room temperature (if stored out of the fridge) or from the fridge — the medication works the same way. Some people find the injection more comfortable at room temperature, but this is optional. Never warm the KwikPen deliberately, and never inject if it's been frozen.
Can I switch between different injection sites within the same week?
Yes, you can, but it's better to rotate sites weekly. If you need to switch sites within a week due to pain or bruising, that's fine — just ensure you're not injecting within 2–3 cm of a recent injection or in a bruised/tender area. Return to your planned rotation schedule the following week.
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.