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: Complete UK Injection Guide
Taking Mounjaro correctly is essential for safety and effectiveness. Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medication licensed in the UK for weight management in adults. This guide covers the practical steps for using your Mounjaro KwikPen, proper injection technique, storage requirements, and what to expect during dose titration. Always follow your prescriber's specific instructions, as individual treatment plans may vary based on your health profile and response to treatment.
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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.
How to Inject Mounjaro: Step-by-Step Technique
Mounjaro comes as a pre-filled KwikPen device containing 4 weekly doses. Each pen delivers one dose automatically when you press the injection button. Here's the correct injection process:
- Choose your injection site: stomach (abdomen), upper thigh, or upper arm. Rotate sites weekly to prevent irritation.
- Prepare the pen: Remove from the fridge 15 minutes before injection to allow it to reach room temperature.
- Clean the skin: Use an alcohol wipe to clean the injection site and allow it to dry.
- Insert needle: Hold the pen perpendicular to your skin and press firmly until you hear a click.
- Press the injection button: Hold in place for 3 seconds, then remove.
- Check the dose window: The dose counter should show zero, confirming the full dose was delivered.
Injecting yourself is straightforward after the first time, though some people prefer a partner or healthcare professional to administer the first dose. Your prescriber or pharmacist can demonstrate the technique before you start.
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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, Timing, and Dose Escalation Schedule
Storage: Unopened pens must be kept in a fridge at 2–8°C. Do not freeze. After first use, you can store the pen at room temperature (below 30°C) for up to 30 days. Keep away from heat and direct sunlight.
Timing: Take your injection on the same day each week. If you miss a dose, inject within 4 days of the missed date. If more than 4 days have passed, skip that dose and inject on your next scheduled day.
Dose escalation: Mounjaro dosing starts at 2.5 mg and increases every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg). Your prescriber will advise on dose progression based on how you respond and any side effects.
- 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 or do I need a healthcare professional?
You can self-inject Mounjaro using the KwikPen after receiving instruction from your prescriber or pharmacist. Many people choose to self-inject once they're confident, though some prefer professional administration for the first dose. Your healthcare provider can demonstrate the technique before you start treatment.
What should I do if I miss a Mounjaro injection?
If you miss a dose, inject within 4 days of the missed date and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next scheduled injection as planned. Never take a double dose. Minimum time between doses should be at least 72 hours.
Does it matter which site I inject Mounjaro into?
Mounjaro can be injected into the stomach (most common), upper thigh, or upper arm. Rotating injection sites each week helps prevent skin irritation, lumps, and discolouration. Avoid injecting into areas with bruises, scars, or tender skin.
How long does Mounjaro stay effective once I start?
Mounjaro is a once-weekly injection, so you need to take it every 7 days to maintain its effects. Individual response times vary; clinical trials showed weight loss benefits building over several weeks of consistent use. Your prescriber will monitor your progress and adjust your dose as needed.
What happens if I accidentally inject more than one dose of Mounjaro?
Accidental double dosing is unlikely with the KwikPen design, but if you do inject twice in quick succession, contact your GP or NHS 111 immediately. Do not attempt to reverse the injection. Your prescriber may need to adjust your next scheduled dose or provide guidance based on your individual circumstances.
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.