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 Use Your Mounjaro Pen Safely
Using your Mounjaro pen correctly is essential for safe and effective weight management treatment. Mounjaro (tirzepatide) comes as a pre-filled KwikPen device that you inject once weekly under your skin. This guide explains step-by-step how to use your Mounjaro pen, where to inject, how to store it, and what to do if you miss a dose. Whether you're accessing Mounjaro through the NHS or privately in the UK, the injection technique and storage requirements are the same.
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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.
Getting Your Mounjaro Prescription in the UK
Mounjaro is a prescription-only weight management injection available in the UK through two routes. On the NHS, access is through specialist tier 3 weight management services — your GP can refer you if you meet eligibility criteria (BMI 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes or hypertension). NICE has approved tirzepatide for NHS specialist services, though availability varies significantly by region and waiting lists can be lengthy. Privately, you can access Mounjaro through a UK-licensed online prescriber following a clinical assessment — this route doesn't require NHS referral and is often faster. Both routes require a valid prescription from a UK-licensed healthcare professional before dispensing. Each KwikPen contains four pre-filled doses (four weeks' supply) and costs vary depending on your dose level and chosen provider.
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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.
Storing and Using Your Mounjaro KwikPen
Storage before first use: Keep your Mounjaro pens in the fridge at 2–8°C in their original carton. Never freeze them. After first injection: You can store the pen at room temperature (below 30°C) for up to 30 days, away from direct sunlight. Each pen is used once weekly — inject the same dose (2.5 mg to 15 mg) on the same day each week. Inject into your stomach, upper thigh, or upper arm (upper arm requires help). Rotate injection sites each week to avoid skin reactions. If you miss a dose by fewer than 4 days, inject as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next dose on the usual day. Never double-dose.
- 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 access Mounjaro through the NHS in the UK?
Yes, Mounjaro (tirzepatide) is approved by NICE for use in NHS specialist weight management services. Access requires a referral from your GP to a tier 3 service. Eligibility criteria include BMI of 30 or above, or 27–29.9 with weight-related conditions. However, NHS availability varies significantly by region and waiting times can be substantial.
What is a Mounjaro KwikPen and how often do I inject?
A KwikPen is a pre-filled injection device containing one dose of Mounjaro. You inject once weekly, always on the same day each week. Each pen is used once then disposed of. Each box contains four pens (one month's supply). You'll start at 2.5 mg and gradually increase to your maintenance dose over several weeks.
Where should I inject my Mounjaro pen?
You can inject into your stomach (abdomen), upper thigh, or upper arm. Most people use their stomach as it's easiest. Rotate your injection site each week to reduce skin irritation. You can inject through thin clothing, but avoid areas that are tender, bruised, or scarred.
How do I store my Mounjaro pens correctly?
Before first use, store pens in the fridge at 2–8°C in their original carton. Never freeze. After your first injection, you can keep the current pen at room temperature (below 30°C) for up to 30 days. Dispose of each pen after use and always keep unused pens refrigerated.
What if I miss my weekly Mounjaro injection?
If you miss your dose but remember within 4 days, inject as soon as possible and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your usual day. Never inject two doses at once. Contact your prescriber if you're unsure.
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.