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
Do You Need To Prime Your Mounjaro Pen Every Time?
Yes, you should prime your Mounjaro KwikPen before every single injection. Priming ensures the needle is properly filled with medication and removes air from the pen, which helps assures you receive the correct dose. This is a critical safety step in the Mounjaro injection process that should never be skipped, regardless of whether you've used the pen before or if it's a new KwikPen device.
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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.
How To Prime Your Mounjaro KwikPen Correctly
Priming your Mounjaro KwikPen is straightforward but essential. Hold the pen upright with the needle pointing upwards, then rotate the dose selector until you see a single unit marked on the dial. Press and hold the injection button until a drop of medication appears at the needle tip — this confirms priming is complete. If no drop appears after pressing the button, repeat the process. Never inject without seeing this drop, as it indicates the medication pathway is properly filled. This priming step takes only a few seconds but is crucial for dose accuracy. After priming, rotate the dose selector to your prescribed dose and proceed with the injection into your chosen site (abdomen, thigh, or upper arm).
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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.
Why Priming Matters For Mounjaro Safety
Priming serves two critical purposes. First, it fills the needle with medication, ensuring your injection contains the full prescribed dose of tirzepatide. Second, it removes air bubbles that could reduce the amount of active medication you receive. Skipping this step risks receiving an incomplete or inconsistent dose, which could affect your treatment effectiveness. The NHS emphasises proper injection technique as essential for prescription medications, and priming is non-negotiable. If you're uncertain about your technique, ask your pharmacist or GP to demonstrate the correct method before your first injection.
- 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 does priming a Mounjaro pen actually do?
Priming fills the needle with medication and removes air from the pen. It ensures you receive your full prescribed dose of tirzepatide and confirms the medication pathway is working correctly. You'll see a small drop of medication at the needle tip when priming is successful.
Can I skip priming if I've already used my Mounjaro KwikPen?
No. Even if you've used the same pen before, you must prime before every injection. Each injection requires a fresh prime to assures a full, accurate dose. Skipping this step may result in under-dosing.
What if no medication drop appears when I try to prime my Mounjaro pen?
If no drop appears after pressing the injection button, try priming again. If it still doesn't work, contact your pharmacist or GP — the pen may be faulty. Never inject without confirming successful priming with a visible medication drop.
How long does it take to prime a Mounjaro KwikPen?
Priming takes only a few seconds. Hold the pen upright, rotate to one unit on the dial, press the injection button until a drop appears, then rotate to your prescribed dose and inject. The entire process should take less than 30 seconds.
Does priming use up any of my Mounjaro dose?
The small amount of medication that appears as a drop during priming is negligible and separate from your actual injection dose. Your prescribed dose remains unchanged — priming simply confirms the pen is working and the needle is filled correctly.
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.