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 Get The Last Bit Out Of Mounjaro Pen
Getting the last dose out of your Mounjaro KwikPen requires proper technique and understanding how the pen works. Unlike some injectors, the Mounjaro KwikPen is designed to deliver exactly four doses (one per week for four weeks) from each pre-filled pen, with minimal wastage if used correctly. This guide explains the correct method, storage requirements, and what to do if you're struggling to extract a full dose from your tirzepatide pen.
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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 Mounjaro Pens Work And Extraction Technique
The Mounjaro KwikPen contains four pre-measured doses, each delivered via a single weekly injection. Each pen is designed to dispense exactly the same amount of tirzepatide regardless of dose strength (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg). Unlike insulin pens where you dial the dose, Mounjaro pens are pre-set — you simply click and inject.
- Ensure the pen has reached room temperature (15–30°C) before use — never inject directly from the fridge
- Check the clear cartridge window for cloudiness or particles; the solution should be clear and colourless
- Remove the outer needle cap, then the inner safety cap
- Pinch skin on stomach, thigh, or upper arm and inject at 90 degrees
- Press the injection button until you hear a click, then hold for 3 seconds to ensure full delivery
If you're not getting a full injection, the most common causes are insufficient pressure on the button, temperature shock (injecting directly from the fridge), or needle blockage. Do not attempt to re-inject — contact your pharmacist or prescriber for a replacement pen if you believe a dose was not fully delivered.
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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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Mounjaro vs Wegovy Pen Design And Dose Extraction
While both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injections available in the UK, they use different pen devices. Mounjaro uses the KwikPen with four pre-measured doses; Wegovy uses a different auto-injection pen where you select your dose. Neither pen is designed to be "emptied" — each contains only the exact amount needed for your prescription doses. The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Both require a UK prescription and clinical assessment of eligibility. Your prescriber will determine which medication suits your circumstances based on your BMI, health conditions, and individual suitability — not based on pen design.
If you're concerned about dose delivery with either medication, speak to your pharmacist or GP rather than attempting to extract additional solution from the pen.
- 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 get more than four doses from a single Mounjaro pen?
No. Each Mounjaro KwikPen contains exactly four pre-measured doses — one per week. The pen is designed to deliver all four doses safely and accurately. Once you've completed four weekly injections, you dispose of the pen and start a new one. Do not attempt to extract additional solution.
What should I do if my Mounjaro pen feels empty but I haven't had four injections?
Contact your pharmacist or GP immediately. If a dose was not fully delivered, you may be entitled to a replacement pen. Never attempt to force more medication out or reuse the pen. Your pharmacist can issue a replacement under standard pharmacy protocols for defective devices.
How should I store my Mounjaro pen to ensure the full dose is available?
Store unopened pens in a fridge at 2–8°C in the original carton. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Always allow the pen to reach room temperature before injecting — cold solution may not dispense properly. Never freeze the pen.
Is the injection technique different for Mounjaro compared to Wegovy?
Yes, slightly. Mounjaro uses a fixed-dose KwikPen (click and inject), whilst Wegovy requires dose selection. Both require proper skin pinching, 90-degree injection angle, and holding the button for the full injection time. Your prescriber will provide specific guidance for whichever medication you're prescribed.
What's the difference between Mounjaro and Wegovy for UK weight loss?
Both are prescription-only weight loss injections available in the UK. Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1 only. Clinical trials showed Mounjaro resulted in ~15–21% average weight loss, Wegovy ~15%. Both are used alongside diet and exercise. Your GP or prescriber assesses which is suitable for your individual health situation.
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.