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 In The Thigh Do You Inject Mounjaro
The upper thigh is one of three approved injection sites for Mounjaro (tirzepatide), alongside your stomach and upper arm. The exact location matters for comfort and effectiveness. This guide explains the precise area of the thigh to use, how to rotate injection sites safely, and what to expect when injecting Mounjaro. Always follow your prescriber's instructions and the patient information leaflet included with your Mounjaro KwikPen.
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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.
The Correct Thigh Injection Site For Mounjaro
The outer thigh is the recommended injection site when using Mounjaro. This means the front and outer area of your upper leg, roughly in the middle of your thigh between your hip and knee. Avoid injecting into areas with bruises, scars, or hardened skin. The thigh is an excellent choice because it is easy to access and rotate between legs week to week. When injecting into your thigh, use a clean, dry area and pinch the skin gently to create a small fold — this helps the needle penetrate to the correct depth beneath the skin. Your Mounjaro KwikPen delivers the injection into the subcutaneous layer (fat tissue just under the skin), not into muscle. Rotate your injection site each week: if you inject the left thigh one week, use the right thigh the next week, then move to your stomach or upper arm. This rotation prevents lipohypertrophy (thickened patches of skin) and ensures consistent absorption of the tirzepatide medication.
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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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Safe Injection Technique And Aftercare
Before injecting, allow your Mounjaro KwikPen to reach room temperature (at least 30 minutes out of the fridge) — this makes the injection less uncomfortable. Hold the pen at a 90-degree angle to your skin, press the injection button, and hold steady for 3 seconds to ensure the full dose is delivered. After injection, you may notice mild redness, bruising, or tenderness at the site — this is normal and usually fades within a few hours. Do not massage the injection area. If you experience severe pain, significant swelling, or signs of infection (warmth, pus, increasing redness), contact your GP or the prescriber who issued your Mounjaro prescription. Store your Mounjaro pen in the fridge between 2°C and 8°C, and always check the expiry date before injecting.
- 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 the same spot on my thigh every week?
No. You must rotate injection sites to avoid lipohypertrophy and ensure consistent absorption. Inject into your left thigh one week, right thigh the next, then switch to your stomach or upper arm. Within each area, space injections at least 2–3cm apart from previous injection sites.
What if I accidentally inject Mounjaro into muscle instead of fat?
If you inject too deeply into muscle, you may experience more pain or bruising. However, the dose will still be absorbed. For future injections, pinch the skin to create a fold and use a gentler angle. Contact your prescriber if you have concerns about an injection technique.
Is the thigh or stomach a better injection site for Mounjaro?
Both are approved sites. The thigh is easy to access and rotate; the stomach is often used as a primary site. Many people alternate between both. Choose what feels most comfortable and accessible for you, and rotate regularly to prevent skin changes.
How long after injecting Mounjaro in my thigh can I exercise?
You can exercise the same day as your injection, though some people prefer to wait a few hours if the injection site feels tender. Light activity is fine immediately; avoid high-impact exercise directly on the injection area for 24 hours.
What should I do if my thigh injection site looks swollen or bruised?
Mild bruising and slight redness are normal and typically fade within days. Apply a cold compress for 10 minutes if needed. Seek medical advice if swelling is severe, spreads, feels warm, or is accompanied by pain, as this could indicate infection or an adverse reaction.
Ready to take control of your weight loss journey?
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