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 Extra Doses From Your Mounjaro Pen
Each Mounjaro KwikPen contains exactly 4 pre-filled doses — enough for 4 weeks of once-weekly injections. You cannot extract extra doses from a single pen, but you can request additional pens from your prescriber if needed. This guide explains how Mounjaro dosing works in the UK, what to do if you run out, and how it compares to Wegovy in terms of dose management and prescription access.
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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.
Understanding Mounjaro Dosing and Pen Supply
Each Mounjaro KwikPen contains exactly 4 individual doses in a single pre-filled auto-injector device. Once you've used all 4 doses, the pen is empty and must be disposed of safely in your local sharps bin — never in household waste.
Dose schedule: You inject once per week, so one pen lasts exactly 4 weeks. Doses typically start at 2.5 mg and increase gradually (up to 5 mg, 7.5 mg, 10 mg, 12.5 mg, then 15 mg maximum) — each dose is held for a minimum of 4 weeks before increasing. If you need additional pens because you're escalating doses or continuing treatment, you must request a new prescription from your prescriber (either NHS specialist services or a private prescriber through an online consultation). There is no way to extract additional doses from a single pen — the device is designed as a sealed, pre-filled injector.
If you're struggling with your current dose or experiencing side effects, contact your prescriber rather than trying to increase frequency or manipulate the pen. Your prescriber can assess whether dose escalation, dose reduction, or additional support is appropriate.
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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: Dose Comparison and Supply
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — a different mechanism to Wegovy. In clinical trials (SURMOUNT data), tirzepatide showed average weight loss of 15–21% of body weight across different doses. Wegovy (semaglutide) is a GLP-1-only agonist; STEP trial data showed approximately 14.9% average weight loss at the maximum dose of 2.4 mg.
Both come in pre-filled pen devices with doses supplied for 4 weeks at a time. Wegovy uses lower doses (0.25 mg up to 2.4 mg) over a longer titration period, while Mounjaro starts at 2.5 mg and escalates to 15 mg. Both require prescription-only access following clinical assessment. If you need additional doses or pens, both require a new prescription from your licensed prescriber — neither pen can be refilled or have extra doses extracted. Your prescriber will assess which medication and dose level is most suitable for your individual health situation.
- 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 extra doses out of my Mounjaro pen if I need more?
No. Each Mounjaro KwikPen contains exactly 4 sealed, pre-filled doses — you cannot extract or refill additional doses. The pen is a single-use device. If you need more doses, request a new prescription from your prescriber (NHS specialist service or private prescriber). Never attempt to manipulate or refill the pen yourself.
What should I do if I run out of Mounjaro before my next prescription?
Contact your prescriber or pharmacy immediately. Do not skip doses or double up on your next injection. If it's been more than 4 days since your missed dose, skip it and continue with your normal weekly schedule. Your prescriber can assess whether you need additional pens or if your dose schedule needs adjusting.
How many pens will I need per month on Mounjaro?
You will need 1 Mounjaro pen per month (4 pens for 4 weeks). Each pen contains 4 once-weekly doses. If you're escalating doses (moving from 2.5 mg to 5 mg, for example), your prescriber will supply pens accordingly — typically one dose at a time during titration, then maintenance doses once you reach your target.
Is Wegovy supplied differently from Mounjaro in terms of pen doses?
Wegovy is also supplied in pre-filled auto-injection pens, but the dosing schedule is different — Wegovy starts at 0.25 mg and escalates over many weeks to 2.4 mg maximum, whereas Mounjaro starts at 2.5 mg. Both require new prescriptions for additional pens; neither pen supplies extra doses. Your prescriber will supply pens based on your titration progress.
Why can't I get more doses from one Mounjaro pen?
The Mounjaro KwikPen is a sealed, pre-filled device designed for safety and accuracy. Each dose is precisely measured (2.5 mg, 5 mg, 7.5 mg, etc.). Opening or attempting to extract additional doses would compromise sterility, accuracy, and safety. Always dispose of empty pens safely in a sharps bin and request a new prescription from your prescriber if you need more.
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