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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Can You Get an Extra Dose From Your Mounjaro Pen?
Each Mounjaro KwikPen is pre-filled with exactly four weekly doses of tirzepatide. Many people ask whether they can extract an extra dose from the pen — the answer is no. The pen is precision-engineered to deliver a specific dose each week, and attempting to extract additional medication could be dangerous. This guide explains how Mounjaro pens work, why you cannot get extra doses, and what to do if you need additional support with your medication in the UK.
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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 KwikPens Are Designed
The Mounjaro KwikPen is a pre-filled, single-use injection device manufactured by Eli Lilly. Each pen contains exactly four weekly doses of tirzepatide, sealed at a precise volume. The pen's mechanism is calibrated to deliver a specific amount of medication each time you press the plunger — you cannot adjust this or extract additional doses.
Why you cannot get extra doses:
- Each dose is measured and sealed within the pen at manufacture
- The needle and cartridge are designed for single-dose delivery only
- Attempting to manipulate the pen could damage the needle, contaminate the medication, or cause an unsafe overdose
- The pen stops dispensing once the four doses are used — there is no mechanism to extract remaining liquid
If you are concerned about your tirzepatide dose or feel it is not working effectively, discuss this with your prescriber during your regular assessment. Your prescriber can review your progress and adjust your dose at the next scheduled escalation point, or investigate whether other factors are affecting your weight loss results.
Clinically BackedWeight Loss
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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.
What To Do If You Need Support With Your Mounjaro Treatment
If you feel your current Mounjaro dose is not meeting your needs, contact your prescriber — whether that is your NHS specialist service, GP, or private online prescriber. They will review your progress using your BMI, weight loss to date, and any side effects you are experiencing. Dose escalations follow a set timetable: you must remain on each dose for at least four weeks before increasing to the next level, with a maximum dose of 15 mg once weekly.
If side effects are preventing you from continuing, your prescriber may suggest staying at your current dose longer, taking a medication break, or switching to an alternative treatment. Never attempt to increase your dose by yourself or extract medication from the pen — always work with your prescriber to adjust your treatment plan safely.
- 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 reuse a Mounjaro pen after the four doses are finished?
No. Once a KwikPen has delivered its four weekly doses, it is empty and should be disposed of safely. Place used pens in a sharps bin or return them to a pharmacy. Do not attempt to refill or reuse the pen — it is designed as a single-use device and cannot be safely reactivated.
What if I accidentally inject more than one dose from my pen?
If you accidentally inject twice in one week, contact your prescriber or NHS 111 immediately. An accidental double dose increases the risk of severe side effects. Your prescriber can advise whether you need medical assessment. Always set a reminder on your phone for your weekly injection day to prevent accidental double-dosing.
How do I know if my Mounjaro dose needs to be increased?
Your prescriber will review your progress at regular intervals, typically every four weeks. Weight loss, side effects, and your overall health context determine whether to escalate. You must stay on each dose for at least four weeks before increasing. If you have concerns about your progress, contact your prescriber rather than trying to adjust the dose yourself.
Is there a way to get Mounjaro at a higher dose more quickly?
No. The tirzepatide dose titration schedule is fixed for safety reasons — you progress from 2.5 mg to 15 mg over a minimum of 20 weeks. Faster escalation increases side effects and safety risks. Your prescriber cannot legally or safely accelerate this timeline, whether through the NHS or private prescription.
What should I do if I lose or damage my Mounjaro pen?
Contact your prescriber immediately to report the lost or damaged pen. If you are prescribed through the NHS, contact your specialist service. If you have a private prescription, contact the online clinic. They can arrange a replacement pen and advise on your next injection date. Do not attempt to extract medication from another pen or delay your weekly dose without guidance.
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.