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
Can You Get Extra Doses From a Mounjaro Pen?
No — Mounjaro KwikPens are designed to deliver exactly one pre-measured dose per week and cannot safely dispense additional doses. Each pen contains enough tirzepatide for precisely four weekly injections, and the mechanism is engineered to stop after the final dose is delivered. Attempting to extract extra medication is dangerous and voids your prescription. This guide explains how Mounjaro pens work, why they're designed this way, and what to do if you need dose adjustments.
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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
Mounjaro pens are single-use, pre-filled devices manufactured by Eli Lilly. Each KwikPen contains exactly four doses of tirzepatide — one for each week of a month. The pen automatically measures and locks after each injection, making it impossible to dispense extra medication.
- Each pen delivers a fixed, pre-measured dose (either 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg)
- The injection mechanism is calibrated to stop after one dose is released
- Attempting to force additional medication risks damaging the pen and injecting an uncontrolled amount
- The tirzepatide formulation inside is sterile only within the sealed pen — exposure compromises safety
Your prescription specifies your exact dose. If you feel your current dose isn't working effectively, discuss dose titration with your prescriber rather than trying to manipulate the pen.
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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 a Dose Adjustment
Mounjaro is designed for gradual dose titration — your prescriber increases your dose in steps, typically every 4 weeks, based on your response and tolerance. If you're experiencing side effects or feel your dose is too low, contact your healthcare provider or prescriber immediately.
- Do not attempt to adjust your dose independently
- Your prescriber can increase your dose at your next appointment if clinically appropriate
- Standard dose escalation follows: 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg (maximum)
- Dose adjustments require at least 4 weeks between increases to assess tolerance and effectiveness
Using our free GLP-1 weight loss calculator can help you understand typical outcomes at different tirzepatide doses based on clinical data, so you know what realistic expectations are for your current prescription.
- 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.
Why can't I get more tirzepatide from my Mounjaro pen?
Each KwikPen is a sealed, pre-measured device containing exactly one week's dose for four weeks. The internal mechanism locks after each injection to prevent accidental overdosing. The tirzepatide formulation remains sterile only inside the unopened pen — extracting extra doses risks contamination and unpredictable dosing, which is medically unsafe.
What happens if I accidentally inject more than one dose from a Mounjaro pen?
Contact your GP or NHS 111 immediately. Overdosing on tirzepatide can cause severe nausea, vomiting, low blood sugar (if you also take diabetes medication), and other serious side effects. You need medical guidance on monitoring and managing any symptoms. Never attempt to extract or self-administer additional doses.
Can I request my GP to prescribe more Mounjaro pens per month?
Your prescriber will only increase your Mounjaro dose (or quantity) if it's clinically appropriate and follows NICE/MHRA guidance. Dose adjustments are carefully timed — typically every 4 weeks — to assess tolerance and effectiveness. If you feel you need a higher dose, discuss this at your next appointment with your healthcare provider.
What's the maximum dose of Mounjaro available in the UK?
The maximum approved Mounjaro dose is 15 mg once weekly. This is the highest tirzepatide dose licensed in the UK. Your prescriber will not exceed this dose, regardless of requests. Clinical trials showed 15 mg delivery approximately 20.9% average body weight loss — individual results vary significantly.
How do I know if my Mounjaro dose is too low?
Signs your dose may need adjusting include: minimal appetite suppression, continued strong cravings, or side effects that have completely resolved (suggesting you've adapted). Discuss these with your prescriber — they can review your progress and plan your next dose increase. Use our free BMI calculator to track your baseline and progress objectively over time.
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.