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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How To Take Mounjaro Injections — Complete UK Guide
Taking Mounjaro (tirzepatide) correctly is essential for safety and effectiveness. This guide walks you through the exact steps for using your KwikPen, choosing injection sites, storing your medication, and what to do if you miss a dose. Mounjaro is a prescription-only, once-weekly injectable medicine that requires a clinical assessment before dispensing. Always follow your prescriber's instructions and the guidance provided with your pen.
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Calculate Your BMI Instantly
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.
Step-By-Step Injection Instructions
Before you inject: Remove your KwikPen from the fridge at least 15 minutes before use to allow it to reach room temperature. Check the solution inside the pen window—it should be clear and colourless. Do not use if it looks cloudy or discoloured. Wash your hands with soap and water. Choose your injection site and clean the skin with an alcohol wipe if desired.
Injecting: Remove the outer needle cap by pulling straight out. Hold the pen firmly with the needle pointing downwards. Pinch the skin at your chosen site to create a fold. Insert the needle at a 90-degree angle fully into the skin. Press the dose button until you hear or feel a click. Keep the pen in place for 3 seconds, then withdraw the needle. You may see a small amount of blood—this is normal. Do not rub the injection site.
After injection: Carefully replace the outer needle cap. Dispose of your used pen in a sharps bin (ask your pharmacy for a free bin if needed). Do not reuse needles or pens.
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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.
Injection Sites and Storage
Where to inject: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to help prevent skin irritation, bruising, or lipodystrophy (lumpy areas of skin). If injecting into your arm, you may need assistance from another person.
Storing your KwikPen: Before first use, store unopened pens in a fridge at 2–8°C in their original carton. Do not freeze—if frozen, the pen cannot be used. After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. Keep away from heat and direct sunlight. Mark the date you opened your pen and dispose of it safely 30 days after first use, even if there is solution remaining.
- 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.
What if I miss a Mounjaro injection?
If you remember within 4 days of your missed dose, inject as soon as you can and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and take your next injection on your regular scheduled day. Never inject twice to make up a missed dose. Always maintain at least 72 hours between injections.
Can I inject Mounjaro myself, or do I need help?
Most people inject Mounjaro into their stomach or thigh independently. If injecting into your arm, you may need assistance from another person. Your prescriber or pharmacist will demonstrate the technique during your initial consultation. It is recommended to watch instructional videos or ask for a demonstration before your first self-injection to build confidence.
How long does a Mounjaro injection take?
The actual injection takes only a few seconds—typically 3–5 seconds from insertion to withdrawal of the needle. However, allow 5–10 minutes to prepare your pen, choose your site, and clean the area. There is no need to rush; take your time to ensure you are comfortable and the injection is done correctly.
What should I do if blood comes out after injecting?
A small amount of blood or leakage at the injection site is normal and nothing to worry about. Simply apply gentle pressure with a clean tissue for a few seconds. You can apply a plaster if desired. Do not rub the area, as this may increase bruising. Bleeding usually stops within 30 seconds. Contact your GP or pharmacist only if bleeding is heavy or persistent.
Is Mounjaro the same as Wegovy or Ozempic?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed for weight management in the UK. Wegovy (semaglutide) is a single GLP-1 agonist—also licensed for weight management. Ozempic also contains semaglutide but is approved only for type 2 diabetes, not weight loss. All three are prescription-only and require clinical assessment. Our GLP-1 weight loss calculator can help estimate potential outcomes with either medication.
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.