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 Do I Inject Mounjaro — Injection Sites and Guidance
Mounjaro (tirzepatide) is a prescription-only, once-weekly injection that you administer yourself at home using a pre-filled KwikPen device. There are several approved injection sites on your body, and rotating between them is important to avoid tissue damage and ensure consistent absorption. This guide explains exactly where to inject Mounjaro, how to rotate sites safely, and answers common questions about self-injection technique.
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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.
Approved Mounjaro Injection Sites
Mounjaro can be injected into three main areas of your body: the stomach (abdomen), the upper thigh, or the upper arm. Each site is equally effective, and rotating between them helps prevent skin irritation, bruising, and lipohypertrophy (thickening of fatty tissue beneath the skin).
- Stomach (abdomen): Most common choice. Inject into the area between your ribs and hipbones, at least 5 cm away from your belly button.
- Upper thigh: Inject into the front or outer part of your thigh, at least 5 cm away from any existing bruises or injection marks.
- Upper arm: Inject into the back of your arm between shoulder and elbow. This site typically requires assistance from another person to inject safely.
All three sites should be rotated each week to reduce the risk of injection site reactions and ensure reliable medication absorption. The NHS and MHRA guidance recommends spacing injections at least 2–3 cm apart.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Site Rotation and Safety Tips
Rotate your injection site every week — if you injected into your left thigh this week, inject into your right thigh, stomach, or arm the following week. This rotation helps prevent repeated tissue damage in the same location. Before injecting, visually inspect the skin for any existing bruising, redness, swelling, or tenderness, and avoid these areas.
Use a fresh needle for every injection — never reuse a needle, even if you're injecting in the same general area. Store your Mounjaro KwikPen in a refrigerator at 2–8°C before first use, and after opening, you can keep it at room temperature (below 30°C) for up to 30 days. Always check that the liquid in the pen is clear and colourless 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 every week?
No. Rotating injection sites weekly is essential to prevent lipohypertrophy, bruising, and skin damage. The MHRA guidance recommends spacing injections at least 2–3 cm apart and using different body areas each week.
Does it hurt to inject Mounjaro?
Most people report minimal discomfort. The needle is very fine (31-gauge), and many describe it as a quick pinch. Pain varies between individuals. If you experience significant pain, check that you're not injecting into scar tissue, bruises, or areas with existing irritation.
Can someone help me inject Mounjaro?
Yes. If you're uncomfortable self-injecting, another person can administer your injection, especially if you choose the upper arm site. They should follow the same sterile technique and site rotation guidelines as you would.
What should I do if I miss a dose or delay my injection?
If you're within 4 days of your scheduled injection day, inject as soon as you remember, then continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your normal day. Minimum time between doses is 72 hours.
Are there any injection sites I should avoid?
Yes — avoid areas with bruising, redness, swelling, lumps, or tender skin. Don't inject within 5 cm of your belly button (abdominal injection) or over scars, moles, or areas where the skin is broken. Always inspect the site visually before injecting.
Ready to take control of your weight loss journey?
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