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 to Inject Mounjaro — Stomach and Other Sites
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. The stomach, or abdomen, is the most commonly used injection site, though your thigh and upper arm are also approved areas. Understanding proper injection technique and site rotation helps minimise discomfort and ensures consistent medication delivery. This guide explains where to inject Mounjaro safely, how to rotate sites, and what to expect during administration.
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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 and Best Practice
Mounjaro can be injected into three areas of the body, each offering specific advantages:
- Stomach (abdomen) — the most commonly used site. Inject into the soft tissue of your lower abdomen, at least 5cm away from your belly button and away from any scars or bruises.
- Upper thigh — an easy-access alternative. You can inject yourself without assistance on either thigh, injecting into the outer thigh area.
- Upper arm — accessible but typically requires assistance from another person to inject safely and accurately into the back of the upper arm.
All three sites are approved by the MHRA and work equally well. Your prescriber or pharmacist will guide you on the best site for your individual circumstances. Rotating injection sites week to week helps reduce tissue irritation and bruising.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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How to Rotate Mounjaro Injection Sites Safely
Site rotation is important because injecting in the same spot repeatedly can cause lipodystrophy (changes in fat tissue). Rotate to a different site each week — for example, stomach one week, left thigh the next, right thigh the week after, then upper arm with assistance. This prevents tissue damage and ensures consistent absorption of the tirzepatide medication.
Always inject at least 5cm away from previous injection marks, scars, bruises, or areas where the skin is hardened, red, or tender. If you notice persistent redness, swelling, or pain at injection sites, contact your GP or the service prescribing Mounjaro. Proper rotation also minimises the "injection fatigue" some users experience with weekly medications.
Before injecting, check that your KwikPen has been stored correctly — refrigerated unopened or at room temperature for up to 30 days after first use. Clean the injection site with an alcohol wipe and let it dry completely. Hold the pen firmly against the skin at a 90-degree angle, press the injection button fully, and hold for 3 seconds to ensure the full dose is delivered. After injection, you can apply gentle pressure with a clean tissue if needed, though bleeding is rare.
- 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 in the same stomach spot every week?
No. Always rotate to a different site each week to avoid lipodystrophy and tissue damage. Alternate between stomach, thighs, and upper arm across different weeks. This helps ensure consistent medication absorption and reduces bruising or irritation at injection sites.
Does it hurt to inject Mounjaro in the stomach?
Most users report minimal pain — the needle is very fine and injection is quick. Some feel a small pinch or pressure. Discomfort varies individually. Injecting into soft tissue away from scars or bruises, using a clean site each week, and allowing the skin to air-dry after cleaning can help minimise any sensation.
What if I get bruising or swelling at my Mounjaro injection site?
Minor bruising is common and usually resolves within days. Avoid injecting in that area for at least 2 weeks. Swelling, redness, or tenderness lasting more than a few days should be reported to your GP or prescribing service. If you develop signs of infection, seek medical advice promptly.
Do I need a prescription to buy Mounjaro in the UK?
Yes. Mounjaro is prescription-only in the UK. You access it either through NHS specialist weight management services (tier 3) or through a private online clinical assessment with a UK-licensed prescriber. A pharmacist or prescriber will guide you on injection technique before you start.
How do I store my Mounjaro KwikPen between injections?
Unopened pens must be refrigerated at 2–8°C in their original carton. After first use, you can store at room temperature below 30°C for up to 30 days. Never freeze. If your pen has been frozen, do not use it. Dispose of the pen after 30 days of first use, even if medication remains.
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