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 Prime Your Mounjaro Pen Every Time?
Yes, you should prime your Mounjaro KwikPen before every single injection. Priming removes air from the needle and ensures you receive the correct dose of tirzepatide. This is a critical safety step outlined in the patient information leaflet and recommended by all UK healthcare providers. If you've been prescribed Mounjaro through an NHS specialist service or private prescriber, your healthcare team should have demonstrated the priming technique during your initial consultation.
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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.
Why Priming Your Mounjaro Pen Is Essential
Priming your Mounjaro KwikPen before each injection is a mandatory safety step. When you prime, you are expelling air from the needle and confirming that the medication is flowing correctly. Without priming, you risk injecting air instead of tirzepatide, which means you may not receive your full prescribed dose.
- Priming takes only 2–3 seconds but is vital for dose accuracy
- The pen should release a small drop of medication when primed correctly
- If no drop appears after priming, do not use that pen — contact your prescriber or pharmacist
- This step applies to every injection, regardless of which dose strength you are using (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg)
MHRA-approved patient guidance and UK prescribing best practice both emphasise priming as a non-negotiable part of safe Mounjaro injection technique. Your healthcare provider should have shown you how to do this before your first dose.
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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.
How to Prime Your Mounjaro Pen Correctly
To prime your Mounjaro KwikPen, hold it upright (needle pointing up) and turn the dose selector to 2 units on the dial. Press and hold the injection button until you see a small drop of medication at the needle tip. This confirms the medication is ready to deliver. Then turn the dial back to your prescribed dose and inject as normal into your stomach, thigh, or upper arm. If you forget to prime before starting your injection, stop, use a fresh pen, and prime again before injecting. Never skip this step.
Our GLP-1 weight loss calculator can help you track expected outcomes once you're on the correct dosing schedule — but priming is essential to ensure each dose is effective.
- 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 happens if I forget to prime my Mounjaro pen?
If you realise you haven't primed before injecting, stop immediately and use a new pen. Priming removes air and confirms medication flow — without it, you may receive no medication or an incomplete dose. Contact your pharmacist or prescriber if you're unsure whether a dose was delivered correctly.
Do I need to prime every single time, even if I've already primed that pen?
Yes. Each time you inject, you must prime that specific pen before use. Priming is a per-injection safety step, not a per-pen step. This applies whether it's your first injection on a new pen or your fourth dose from the same pen.
What should I see when I prime correctly?
A small liquid drop should appear at the needle tip when you prime. If no drop appears after pressing the button, the pen may be faulty. Do not use it — contact your pharmacy or prescriber for a replacement pen.
Can I prime my Mounjaro pen at room temperature or does it need to be cold?
Your Mounjaro should be at room temperature before injection (if it's been refrigerated, take it out 30 minutes before use). You can prime at room temperature. The medication should flow clearly when you prime; if it appears cloudy or won't flow, do not use the pen.
Is priming the same for all Mounjaro doses?
Yes. Whether you're using a 2.5 mg starter dose, a 5 mg maintenance dose, or the maximum 15 mg dose, the priming technique is identical. Always turn the dial to 2 units, press until a drop appears, then adjust to your prescribed dose and inject.
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.