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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Mounjaro Injection Sites: Stomach, Thigh & Arm Explained
Mounjaro (tirzepatide) is a once-weekly weight loss injection available in the UK through NHS specialist services or private prescription. The stomach is the most common injection site, though the upper thigh and upper arm are also suitable. Understanding where and how to inject, along with UK access routes and costs, helps you prepare if Mounjaro is prescribed to you. This guide covers injection technique, site rotation, and what to expect from the KwikPen device.
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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.
Where to Inject Mounjaro: Approved Sites and Technique
Mounjaro comes in a pre-filled KwikPen device and is injected under the skin (subcutaneously) once per week. Three injection sites are approved:
- Stomach (abdomen) — the most commonly used site. Inject at least 5cm away from your belly button, varying the exact spot each week to avoid repeated injections in the same place.
- Upper thigh — a practical alternative, easily accessible for self-injection. Avoid the inner thigh.
- Upper arm — suitable but typically requires assistance from another person to inject safely.
Always rotate your injection site each week to reduce irritation and allow the skin to recover. You can inject at any time of day, with or without food. The injection itself takes only a few seconds. If you're unsure about injection technique, your prescriber or pharmacist will provide detailed guidance and demonstration before you start.
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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.
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UK Cost and Access: NHS Specialist Services vs Private Prescription
NHS access: Mounjaro is available through tier 3 and 4 NHS specialist weight management services following referral from your GP. NICE has approved tirzepatide (Mounjaro) for use within NHS services, though regional availability varies significantly and waiting lists can be lengthy in many areas.
Private prescription cost: Through GPhC-registered online pharmacies, Mounjaro costs typically range from £130–£280 per month depending on the dose. A private online clinical assessment (usually £15–£50) is required before prescription. Private access is faster than NHS but requires you to cover the full cost. Use our free GLP-1 weight loss calculator to estimate potential results at different doses before committing to treatment.
- 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 stomach if I have excess belly fat?
Yes. The stomach area has sufficient subcutaneous tissue for injection, even with excess weight. Varying injection sites across the abdomen each week helps prevent repeated trauma to the same area. Your prescriber or pharmacist can advise on technique specific to your body composition.
What happens if I inject Mounjaro in the wrong place?
If you accidentally inject into muscle or at a non-approved site, contact your pharmacist or GP for guidance. Injecting into muscle may cause discomfort but is unlikely to cause serious harm. Always confirm approved sites with your prescriber before self-injecting.
How much does Mounjaro cost on private prescription in the UK?
Private Mounjaro costs range from approximately £130–£280 per month depending on your dose (2.5mg to 15mg). NHS access through specialist services is free but involves waiting lists and regional variation. Online private consultations typically cost £15–£50 before prescription is issued.
Can I switch from Mounjaro to Wegovy if injection sites become irritated?
Both medications use similar injection sites (stomach, thigh, arm). Switching requires discussion with your prescriber. Site rotation and good injection technique usually resolve irritation. Ensure you're rotating sites each week and allowing at least one week between injections at the same location.
Do I need to be referred to the NHS before accessing Mounjaro privately?
No. Private prescription Mounjaro is available without NHS referral through online clinical assessment. You do not need to be on an NHS waiting list or referred to a tier 3 service to access private prescription. However, your GP should be informed for safety reasons.
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