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 Can You Inject Mounjaro? A UK Guide to Injection Sites
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. In the UK, there are three approved injection sites on your body where you can safely inject Mounjaro. Knowing the correct areas to inject, how to rotate between sites, and proper injection technique is essential for getting the most from your prescription. This guide covers everything you need to know about Mounjaro injection sites according to UK prescribing guidance.
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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.
Approved Mounjaro Injection Sites in the UK
Mounjaro can be injected into three areas of your body:
- Stomach (abdomen) — the most commonly used site, approximately 5cm away from the belly button, avoiding scars
- Upper thigh — the outer thigh area, roughly halfway between hip and knee
- Upper arm — the back of the upper arm, between shoulder and elbow (requires assistance or use of an auto-injector device)
Each site should be a minimum of 2.5cm away from previous injection locations. The stomach remains the most popular choice as it is easiest to access independently. When injecting into the upper arm, you may find it helpful to have a partner assist, or use the KwikPen's auto-injection feature. Always inspect the injection site before administering your dose — avoid areas with redness, bruising, lumps, or dimpling, as these may indicate lipohypertrophy (thickened tissue) or other reactions requiring medical attention.
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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.
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Rotating Your Injection Sites and Preventing Complications
Site rotation is important for preventing tissue damage and improving medication absorption. Change your injection site each week — for example, if you injected into your stomach one week, choose your thigh or upper arm the next week. Within each site (stomach, thigh, or arm), vary the exact location by at least 2.5cm each time. Rotating sites reduces the risk of lipohypertrophy, lipoatrophy (dimpling), and injection site reactions like redness or bruising.
If you notice any lumps, thickening, dimpling, or persistent redness at injection sites, inform your GP or prescriber immediately. These may require temporary avoidance of that area whilst the tissue recovers. Using our free GLP-1 weight loss calculator can help you track your progress alongside understanding your medication administration routine.
Store your Mounjaro KwikPen in a refrigerator at 2–8°C if unopened. After first use, it can be kept at room temperature below 30°C for up to 30 days. Before each injection, allow the pen to reach room temperature for at least 15 minutes. Never inject cold medication directly from the fridge. Inspect your dose window for air bubbles — if present, gently tap the pen and follow instructions in your patient information leaflet. Always check the expiry date and ensure the medication is clear and colourless before injecting. If you have any concerns about your injection technique, ask your GP or prescriber for a demonstration.
- 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 different body parts each week?
Yes — rotating between your stomach, thigh, and upper arm each week is recommended. Within each site, change the exact location by at least 2.5cm. This rotation helps prevent tissue damage and improves medication absorption over time.
What should I do if I have redness or bruising at my injection site?
Minor redness or bruising can occur and typically resolves within a few days. However, if you notice persistent redness, lumps, thickening, or dimpling, inform your GP or prescriber. Avoid injecting into that area until it has fully recovered. This may indicate lipohypertrophy requiring rest.
Is it better to inject Mounjaro in the stomach or thigh?
Both are equally effective injection sites. The stomach is most common because it's easiest to access independently. Choose whichever site you find most comfortable and convenient. Rotating between sites is more important than choosing one specific location.
Do I need to warm my Mounjaro pen before injecting?
Yes. If your KwikPen has been stored in the fridge, remove it 15 minutes before injection to allow it to reach room temperature. Never inject cold medication directly from refrigeration — this can cause discomfort and affects medication delivery.
What if I can't inject into my upper arm by myself?
The upper arm is the most difficult site to access independently. You can ask a partner to assist with the injection, or choose your stomach or thigh instead, which are easier to self-inject. Your prescriber can demonstrate the technique if needed.
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