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
How to Administer Mounjaro Safely
Mounjaro (tirzepatide) is a prescription-only injection that you administer yourself once weekly using a pre-filled KwikPen device. This guide walks you through the exact steps to inject safely, where to inject, how to store your pens, and what to do if you miss a dose. Always follow your prescriber's instructions, and speak to your GP or pharmacist if you have any concerns about your injection technique.
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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 injecting, wash your hands thoroughly and allow the KwikPen to sit at room temperature for 15 minutes if it has been refrigerated — this makes the injection more comfortable. Remove the pen from its carton and check that the liquid inside is clear and colourless; if it appears cloudy or discoloured, do not use it. Pinch a fold of skin at your chosen injection site, hold the pen at a 90-degree angle to the skin, and push the needle straight in until you hear or feel a click. Hold steady for 3 seconds to ensure the full dose is delivered, then withdraw the needle and release the pinched skin. Never rub the injection site afterwards — light pressure is fine. Each KwikPen contains four weeks of doses (one injection per week), so after four injections, dispose of the pen safely in a sharps bin and start a new one.
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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.
Where to Inject and Storage Guidelines
Injection sites: Inject into your stomach (abdomen — ideally 5cm away from your belly button), upper thigh, or upper arm. Rotate your injection site each week to prevent lipodystrophy (thickened or indented skin). Storage: Unopened pens must be kept refrigerated at 2–8°C in their original carton and away from light. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Never freeze Mounjaro — if it has been frozen, discard it. Always dispose of used pens in a designated sharps bin, not in household waste.
- 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 should I do if I miss a Mounjaro injection?
If you remember within 4 days of your scheduled dose, inject as soon as possible and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your regular scheduled day. Never take a double dose. Maintain at least 72 hours (3 days) between injections.
Can someone else help me inject Mounjaro?
Yes, a family member or carer can help you inject, particularly if you're using the upper arm site. Your GP or pharmacist can show you both the correct injection technique. The pen should always be held at a 90-degree angle to the skin.
What if I see blood or fluid after injecting?
A small amount of blood or liquid at the injection site is normal and not a cause for concern. Simply apply light pressure with a clean tissue for a few seconds. If bleeding is heavy or persistent, contact your GP. Bruising is also common and usually fades within a few days.
How do I know if my Mounjaro pen is working correctly?
After injecting, you should hear or feel a click and the dose counter on the pen will decrease by one. If the pen doesn't click or the counter doesn't change, the injection may not have been delivered — contact your pharmacist or GP for guidance.
Can I travel with Mounjaro?
Yes. Carry unopened pens in their original carton in an insulated travel bag with an ice pack (keep at 2–8°C). Once you've started a pen, it can stay at room temperature for up to 30 days. Take your prescription paperwork and a letter from your GP or pharmacist confirming you're using the medication for weight management.
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.