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 on your thigh to inject Mounjaro safely
Mounjaro comes in a pre-filled KwikPen device and can be injected into the upper thigh, along with your stomach (abdomen) and upper arm. The upper thigh is one of the three approved injection sites in the UK. This guide explains exactly where on your thigh to inject, how to rotate sites safely, and what to avoid. Always follow the instructions in your KwikPen patient information leaflet, and if you're unsure, ask your GP or pharmacist for guidance before your first injection.
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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.
Safe thigh injection sites and technique for Mounjaro
The upper outer thigh is the recommended injection site on the leg. Specifically, aim for the area on the outer surface of your thigh, roughly halfway between your hip and knee. This region has good subcutaneous fat and is away from nerves and major blood vessels. Inject at least 2 inches (5 cm) away from your kneecap and avoid the inner thigh where skin is more sensitive. Each KwikPen delivers a once-weekly dose of tirzepatide, and your prescriber will guide you through the starting dose and any titration schedule. When injecting, pinch the skin gently, insert the needle at a 90-degree angle, and hold for 3 seconds after pressing the injection button. Never inject into areas that are bruised, tender, red, or hardened from previous injections.
- Use the outer thigh — away from bone and inner leg
- Inject halfway between hip and knee for best accessibility
- Maintain 2-inch spacing from previous injection sites
- Rotate between left and right thigh each week if using thigh only
- Never reuse a needle or inject through clothing
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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 injection sites and avoiding complications
Site rotation is essential to prevent lipodystrophy (thickened or sunken skin patches) and ensure proper medication absorption. Mounjaro is typically injected once per week, so you should rotate between your three approved sites: stomach, upper thigh, and upper arm. Many UK patients use a simple rotation pattern: Week 1 thigh, Week 2 stomach, Week 3 arm, then repeat. Within each site, space injections at least one finger-width apart — use the weight converter and GLP-1 calculator on this page to track your progress alongside careful injection hygiene. After injection, you may see mild redness, bruising, or swelling at the injection site; this usually settles within a few hours. If you experience severe pain, persistent swelling, or signs of infection (increasing warmth, redness, or pus), contact your GP.
- 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. Repeated injections in the same spot can cause lipodystrophy (thickened or depressed skin). Rotate between your three approved sites (stomach, thigh, arm) and within each site, space injections at least one finger-width apart. This ensures better medication absorption and reduces skin complications.
How deep should the needle go when injecting into the thigh?
The KwikPen needle is pre-set and designed for subcutaneous injection — it automatically reaches the correct depth. You should feel slight resistance as the needle enters the skin, then push the button and hold for 3 seconds. The entire injection takes about 5–10 seconds. Never try to adjust needle depth yourself.
What if I hit a nerve or blood vessel when injecting my thigh?
Hitting a major nerve or blood vessel is very unlikely if you follow the guidance: inject into the outer thigh, away from the inner leg, and avoid bony areas. If you experience sharp pain, numbness, or significant bleeding, stop, apply pressure, and contact your GP. Most injection site sensations are normal mild discomfort.
Is the upper thigh better than the stomach for Mounjaro injections?
Both are equally effective sites. Some UK patients prefer the thigh for privacy or convenience. The stomach may absorb medication slightly faster, but the difference is minimal. Choose based on comfort and accessibility. Rotating between all three sites (stomach, thigh, arm) is recommended to prevent skin complications.
What should I do if my thigh injection site becomes red, swollen, or bruised?
Mild redness and bruising are normal and usually resolve within hours. Apply a cold compress if uncomfortable. However, if redness spreads, swelling persists beyond 24 hours, or you see signs of infection (warmth, pus, fever), contact your GP. Never inject into a bruised or infected area — choose a different site.
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