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 — Safe Injection Points
Mounjaro (tirzepatide) is a once-weekly weight loss injection that you administer yourself at home using a pre-filled KwikPen. The medication can be injected into three main areas of your body: your stomach (abdomen), upper thigh, or upper arm. Rotating your injection site each week helps prevent skin irritation and ensures consistent medication absorption. This guide explains the correct injection points, technique, and what to expect when using Mounjaro as part of your weight management journey following NHS or private prescription.
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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 and Rotation
Mounjaro can be injected into three safe areas:
- Stomach (abdomen) — the most commonly used injection site. Inject at least 5cm (2 inches) away from your belly button and avoid areas with scars, bruises, or hard lumps.
- Upper thigh — on the front or outer side of your thigh. This site is convenient and easy to access yourself.
- Upper arm — on the back of your arm (triceps area). This site requires assistance from another person to inject safely.
Each week, you should rotate your injection site to prevent lipohypertrophy (thickened skin) and ensure even absorption of the tirzepatide. If you're injecting into your stomach weekly, choose a different spot each time — for example, upper left one week, lower right the next. Avoid injecting into the same area more than once per month. Never inject into areas with active skin conditions, scars, bruises, or skin that feels hard or lumpy.
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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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Injection Technique and What to Expect
Before injecting, wash your hands and let the KwikPen reach room temperature if stored in the refrigerator (at least 30 minutes). Pinch the skin gently at your chosen injection site, insert the needle at a 90-degree angle, and hold the pen in place for 3 seconds whilst the dose is delivered — you'll hear a click. After injection, some people experience mild reactions like redness, bruising, or tenderness, which typically fade within a few hours. These injection site reactions are common and usually not serious. If you experience severe pain, significant swelling, or signs of infection, contact your GP immediately. Store used pens safely in a sharps bin and dispose of them according to your local pharmacy's instructions.
- 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. You should rotate your injection site each week to prevent skin thickening and ensure consistent medication absorption. You can inject into the same general area (e.g., stomach) but choose a different spot within that area. Avoid injecting into the exact same location more than once per month. Rotating between stomach, thigh, and upper arm helps prevent complications.
What should I do if I get a bruise or swelling after injecting Mounjaro?
Mild bruising and swelling are common after injection and usually resolve within hours to days. Apply a cold compress immediately after injection if desired. Avoid injecting into that area again for at least one month. If swelling is severe, doesn't improve within 48 hours, or you notice signs of infection (warmth, pus, increasing redness), contact your GP or call NHS 111 for advice.
Do I need assistance to inject Mounjaro?
Most people can inject into their stomach or thigh independently without assistance. However, if you choose to use your upper arm as an injection site, you'll need another person to inject you safely, as the angle and access make self-injection difficult. Your GP or practice nurse can show you the correct technique during your initial appointment.
Can I inject Mounjaro through my clothes?
No. You must inject into bare skin. Remove or pull up your clothing to expose the injection site completely. Never attempt to inject through fabric, as this reduces accuracy and increases infection risk. Once you've chosen and cleaned your injection site, you're ready to inject.
What's the minimum time between Mounjaro injections?
Mounjaro must be injected at least 72 hours (3 days) apart to maintain safe dosing intervals. The standard schedule is once per week on the same day each week — for example, every Monday. If you're more than 4 days late with your injection, skip that dose and take your next dose on your normal scheduled day. Never take a double dose to make up for a missed 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.