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 Administer Mounjaro: Step-by-Step UK Guide
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. If you've been prescribed this prescription-only weight management medication, it's important to understand the correct injection technique, where to inject, and how to store your pens safely. This guide walks through the practical steps for self-administering Mounjaro at home, helping you use the medication confidently and effectively alongside a reduced-calorie diet and increased physical activity.
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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.
Mounjaro Injection Sites and Administration Technique
Mounjaro is injected once weekly, always on the same day each week if possible. The KwikPen contains enough medication for one 4-week supply (4 injections). You can inject Mounjaro into three main areas: the stomach (abdomen), the outer thigh, or the upper arm (though the upper arm requires assistance from another person). Rotate your injection site each week — do not inject in exactly the same spot repeatedly, as this can cause lumps or skin irritation. Before injecting, check that the medication is clear and colourless (or slightly amber). If it appears cloudy, discoloured, or contains particles, do not use it and contact your prescriber. Pinch a fold of skin, insert the needle at a 90-degree angle, and press the injection button slowly until you hear or feel a click. Keep the pen against your skin for 6 seconds after the click to ensure the full dose is delivered. The needle is very fine and the injection typically takes only a few seconds.
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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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Storage, Aftercare, and Missed Doses
Storage before first use: Keep unopened KwikPens in a refrigerator at 2–8°C in the original carton. Do not freeze — if frozen, discard the pen. After first use, you can store the pen at room temperature (below 30°C) for up to 30 days, then discard it. Aftercare: You may experience mild redness, bruising, or soreness at the injection site — this is normal. Apply a cool compress if needed. Missed doses: If you miss your weekly injection, administer it within 4 days of the scheduled date and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on the normally scheduled day. Never take a double dose. Always allow at least 72 hours (3 days) between injections. Use our GLP-1 weight loss calculator to track projected outcomes alongside consistent injection timing.
- 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 myself at home?
Yes, Mounjaro is designed for self-administration at home using the pre-filled KwikPen. You will receive training from your prescriber or a nurse on the first injection. After that, most people inject themselves. If you're uncomfortable self-injecting, a healthcare professional, family member, or carer can administer it for you.
Which injection site should I choose?
The stomach (abdomen) is the most common site and is easiest to access yourself. The outer thigh is also straightforward. The upper arm requires assistance. Rotate sites each week to avoid skin irritation and lipohypertrophy (lumps under the skin). All three sites work equally well.
What if I inject into scar tissue or a lump?
Avoid injecting into areas with scars, bruises, or lumps. If you notice a lump developing (lipohypertrophy), switch to a different injection site. If lumps persist or cause pain, inform your GP or prescriber — they may need to review your injection technique.
How long does a Mounjaro injection take?
A Mounjaro injection typically takes just a few seconds. Hold the pen against your skin for 6 seconds after the click to ensure full dose delivery. The entire process, including preparation, is usually under 1 minute.
What should I do if I experience severe pain or bleeding after injection?
Minor redness, bruising, or mild soreness is normal. However, if you experience severe pain, significant bleeding, or signs of infection (increasing redness, warmth, pus), contact your GP or prescriber immediately. These reactions are uncommon but require medical attention.
Ready to take control of your weight loss journey?
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