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 Should You Inject Mounjaro — Stomach Or Thigh?
Both the stomach and thigh are approved injection sites for Mounjaro (tirzepatide), and neither is inherently "better" — the choice depends on your comfort, accessibility, and skin condition. The NHS and UK prescribing guidance support rotation between approved sites. Your prescriber will advise on the best approach for your individual circumstances. Understanding injection site options helps you use your Mounjaro KwikPen safely and consistently throughout your treatment.
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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 And NHS Guidance
Approved injection sites for Mounjaro in the UK are:
- Stomach (abdomen) — most commonly used, often preferred for convenience and visibility
- Upper thigh — accessible and easy to reach, good for rotation
- Upper arm — approved but requires assistance from another person to inject safely
The NHS and MHRA-approved product information emphasise rotating your injection site each week to reduce the risk of lipodystrophy (changes to skin or fatty tissue at injection sites). This rotation pattern helps prevent irritation, bruising, and tissue damage. Never inject into the same spot twice in a row. The stomach is the most popular choice because it provides good visibility and access without assistance, but clinical effectiveness is identical across all three approved sites. Your prescriber or pharmacist will demonstrate the safest technique for your chosen site when you start Mounjaro treatment.
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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.
Practical Tips For Safe And Comfortable Injections
Choose injection sites based on what feels most comfortable and sustainable for you long-term. If you prefer not to inject yourself (particularly in the arm), the stomach or thigh offer independence. Ensure the injection site is clean and dry before injecting. Allow your Mounjaro pen to warm to room temperature for 15–30 minutes before use if it has been refrigerated — this can reduce injection discomfort. Pinch the skin gently when injecting into the stomach, and allow at least 2 centimetres between injection sites each week. If you develop redness, pain, or swelling at an injection site, contact your GP or pharmacist. Your prescriber or nurse can review your injection technique to ensure you are using your KwikPen correctly and minimising discomfort.
- 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 stomach or thigh injection better for Mounjaro results?
Neither is better for results — tirzepatide is absorbed the same way from all approved sites. The choice is entirely about your comfort and accessibility. Rotation between sites reduces skin irritation and lipodystrophy risk, which is why varying injection sites each week is recommended by UK prescribers.
Can you inject Mounjaro in the same spot twice?
No. The NHS and product guidance recommend rotating your injection site each week to prevent lipodystrophy (tissue changes) and bruising. Leave at least 2cm between injection sites. Never inject into the same spot within 7 days.
What if I feel pain or bruising at my Mounjaro injection site?
Mild redness or bruising is normal, but severe or persistent pain warrants review. Contact your GP or pharmacist if you develop swelling, hardening, or signs of infection. Your prescriber can check your injection technique and recommend ways to reduce discomfort, such as allowing the pen to warm up before injection.
Is Mounjaro injection easier in the stomach or thigh?
Most people find the stomach easier because it is visible and does not require assistance. However, the thigh is equally accessible and many find it more convenient for clothing. The arm requires help from another person. Choose based on what feels easiest for you to sustain weekly.
How do I know if I have lipodystrophy from Mounjaro injections?
Lipodystrophy appears as lumps, dimples, or hollows in the skin at injection sites. It develops from repeated injections in the same spot. Proper site rotation prevents it. If you notice changes, tell your GP or pharmacist — they can advise on technique adjustments and whether specialist assessment is needed.
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.