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 Is the Most Effective Place to Inject Mounjaro?
The most effective injection sites for Mounjaro are the stomach (abdomen), upper thigh, and upper arm — all three areas are equally effective when used correctly. Mounjaro using the KwikPen comes pre-filled with tirzepatide and works the same way regardless of injection location. The key to effectiveness is rotating injection sites each week to prevent irritation and ensure consistent absorption. Your prescriber will guide you on proper technique during your initial clinical assessment.
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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.
Mounjaro Injection Sites vs Wegovy: How They Compare
Both Mounjaro and Wegovy use the same recommended injection sites — stomach, upper thigh, and upper arm — making them comparable in terms of administration flexibility. However, there are important differences in how they're delivered. Mounjaro is a once-weekly injection using the KwikPen device, while Wegovy is also once-weekly but in a different pre-filled pen design. The tirzepatide in Mounjaro (a dual GIP and GLP-1 receptor agonist) may be absorbed slightly differently than semaglutide in Wegovy (GLP-1 only), though this doesn't affect which injection site is "best" — clinical evidence from the SURMOUNT trial for Mounjaro and STEP trials for Wegovy shows both deliver effective results across all three approved sites when rotation is practiced consistently.
- Stomach (abdomen) — most commonly used; easy to self-inject
- Upper thigh — accessible and convenient for self-injection
- Upper arm — typically requires assistance from another person
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Finding Your Best Injection Site and Rotation Schedule
The most effective approach is choosing the site that feels most comfortable and sustainable for you, then rotating weekly to prevent lipohypertrophy (thickened skin). Start with your stomach if self-injecting alone — it's the most accessible. Your UK prescription will include detailed instructions, and your prescriber or pharmacist can demonstrate the correct technique. Keep sites at least 2–3 cm apart, never inject into areas of thickened, lumpy, or damaged skin, and allow skin to dry fully after alcohol swab cleaning. Mounjaro eligibility in the UK requires clinical assessment to confirm you're suitable, but once approved, rotating injection sites is a straightforward part of your weekly routine that ensures consistent drug absorption and reduces injection-site reactions.
- 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.
Does it matter which injection site I use for Mounjaro?
No — stomach, thigh, and upper arm are all equally effective for Mounjaro absorption. What matters is rotating sites weekly to prevent skin irritation and ensure consistent delivery. Choose whichever location feels most comfortable and accessible for you to self-inject safely.
Can I inject Mounjaro in the same spot every week?
No, rotating sites each week is essential. Injecting in the same spot repeatedly can cause lipohypertrophy (thickened or lumpy skin patches) and may affect drug absorption. Always space injections at least 2–3 cm apart and avoid previously used areas.
Is Mounjaro injection technique the same as Wegovy?
The injection sites are the same (stomach, thigh, upper arm), but the devices differ — Mounjaro uses the KwikPen, Wegovy uses a different pre-filled pen design. Both are once-weekly. Your prescriber will demonstrate the correct technique for whichever medication you've been prescribed.
Why is rotating injection sites important?
Rotating sites prevents lipohypertrophy (skin thickening), reduces injection-site pain and bruising, and ensures even drug absorption. Consistent rotation helps maintain the effectiveness of your tirzepatide treatment and minimises local skin reactions.
Can my GP show me how to inject Mounjaro correctly?
Yes. Your prescriber or pharmacist will provide detailed instructions and a demonstration before you leave with your first KwikPen. Most people find the pre-filled pen simple to use after one demonstration. Always ask if you're unsure about technique.
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.