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: Best Sites and Technique
Mounjaro (tirzepatide) comes in a pre-filled KwikPen device and can be injected into three main body sites. The most effective place to inject Mounjaro depends on rotating between approved sites to avoid irritation and maintain consistent absorption. This guide covers the three authorised injection locations, how to rotate between them, and how Mounjaro's injection method compares to Wegovy when considering which prescription weight loss medication might suit you best.
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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 Rotation Technique
Mounjaro can be injected into three authorised body locations, and rotating between them is essential for consistent absorption and to reduce injection site reactions:
- Stomach (abdomen): The most commonly used site. Inject into the fatty tissue around your stomach, avoiding the 5cm area around your belly button (navel). This site typically offers reliable absorption and is easiest to access yourself.
- Upper thigh: The front or outer area of your upper leg. This is equally effective and provides good absorption. Rotating between left and right thigh is recommended.
- Upper arm: The back of your upper arm, in the fatty area. This site typically requires assistance from another person to inject safely and accurately.
Rotate your injection site each week to avoid repeated trauma to the same area. This helps prevent lipohypertrophy (thickened skin), bruising, and injection site discomfort. There is no clinical evidence that any single site is more effective than others for weight loss — absorption is consistent across all three authorised locations when rotated properly.
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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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Mounjaro vs Wegovy: Injection Method Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) use similar subcutaneous injection techniques with identical approved injection sites: stomach, upper thigh, and upper arm. Both medications come in pre-filled pen devices designed for once-weekly self-injection. The key difference lies in their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 only. In clinical trials (SURMOUNT vs STEP studies), Mounjaro showed average weight loss of approximately 15–21% of body weight, whilst Wegovy showed around 14.9% on average. Your prescriber will assess which medication is more suitable based on your individual health profile, medical history, and any weight-related conditions. Use our GLP-1 weight loss calculator to explore potential outcomes with either medication, and our BMI calculator to establish your current health baseline before discussing options with 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 site every week?
No. You must rotate your injection site each week across the three approved locations (stomach, upper thigh, upper arm) to prevent skin thickening, bruising, and inconsistent absorption. Always leave at least 2–3cm between injection sites within the same general area.
Is the stomach the most effective Mounjaro injection site?
All three authorised sites (stomach, thigh, and arm) offer equally effective absorption when rotated properly. The stomach is most commonly used because it's easiest to access yourself, but there's no clinical evidence it absorbs faster or better than the thigh or arm.
What happens if I inject Mounjaro into muscle instead of fatty tissue?
Mounjaro is designed for subcutaneous injection (into fatty tissue beneath the skin). Injecting into muscle may cause discomfort and could affect absorption. Always pinch the skin slightly to ensure you're injecting into the fatty layer, not muscle.
How far apart should Mounjaro injection sites be?
Within the same body area (e.g., stomach), space injections at least 2–3cm apart. You can use different areas each week — for example, stomach week one, right thigh week two, left thigh week three, upper arm week four, then repeat the rotation.
Does injection site location affect how quickly Mounjaro works for weight loss?
No. All three authorised injection sites have similar absorption rates and blood concentration levels. Weight loss outcomes depend on the dose, your diet, physical activity, and individual metabolism — not which site you choose, as long as you rotate correctly.
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.