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 Can I Inject Mounjaro?
Mounjaro (tirzepatide) is injected into specific areas of your body using the pre-filled KwikPen device. The main injection sites are your stomach, upper thigh, and upper arm — though stomach injections are most common. Rotating your injection site each week helps prevent skin irritation and ensures consistent absorption. This guide explains the approved sites, how to rotate safely, and practical tips for self-injection with your prescription KwikPen.
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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:
- Stomach (abdomen): The most commonly used site. Inject into the soft tissue on your belly, at least 2 inches away from your belly button. This area has good fatty tissue for subcutaneous injection.
- Upper thigh: The outer area of your thigh (front or side). Pinch the skin gently to create a soft tissue fold before injecting.
- Upper arm: The back of the upper arm (triceps area). This site typically requires assistance from another person, as it can be difficult to self-inject accurately here.
All three sites work equally well. Your prescriber or nurse will show you how to inject safely during your first appointment. The KwikPen device is designed for easy, one-handed use at stomach and thigh sites.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Rotating Your Injection Site Each Week
Rotating injection sites is important to prevent:
- Skin irritation and redness
- Bruising or swelling at repeated sites
- Lipohypertrophy (thickened fatty tissue)
- Reduced absorption of the medication
A simple rotation pattern: Week 1 stomach, Week 2 left thigh, Week 3 right thigh, Week 4 upper arm (with help), then repeat. Always inject at least 1 cm away from previous injection sites. If you notice persistent redness, lumps, or pain at any site, mention this to your GP or prescriber. You can use our free BMI calculator to track how your weight responds as you progress through your tirzepatide dose titration.
- 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 muscle instead of fat?
No. Mounjaro is designed for subcutaneous injection — into the fatty tissue layer under your skin, not into muscle. Injecting into muscle may cause pain and affect how your body absorbs the medication. Always pinch the skin to ensure you're injecting into fat tissue, not muscle.
What should I do if I hit a blood vessel during injection?
If you see blood when you pull out the needle, apply gentle pressure with a cotton ball or tissue for a few moments. A small amount of bleeding is normal and not dangerous. If bleeding is heavy or won't stop after 5 minutes, contact your GP. This is rare with proper injection technique.
Can I use the same injection site twice in one week?
No. Mounjaro is a once-weekly injection, so you inject one dose per week. Always use a different site each week to allow the previous site to recover. Using the same site repeatedly increases risk of bruising, irritation, and reduced medication absorption.
Is injecting into my stomach painful?
Most people find Mounjaro injections minimally painful. The needle is very fine (29 gauge). Some people feel a slight sting; others feel nothing. Stomach injections are generally less painful than thigh sites because there's more fatty tissue. Pain varies between individuals and often decreases with practice.
What if I can't reach my injection site easily?
Stomach and outer thigh are easiest for self-injection. If you have mobility issues, your upper arm is an option but requires a carer or partner to help. Talk to your prescriber about alternative sites or ask a nurse to show you techniques that work for your situation. Never rush an injection.
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.