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
How Many ML Is In A Mounjaro Pen?
A Mounjaro KwikPen contains a total of 3 ml of solution and holds 4 pre-filled weekly doses. Each dose ranges from 2.5 mg to 15 mg of tirzepatide (the active ingredient), delivered as a once-weekly subcutaneous injection. The exact volume per individual injection varies depending on which dose strength you're prescribed, but the pen itself always delivers a full 4-week supply. Understanding your Mounjaro UK prescription details, including dose titration schedules and how this differs from Wegovy alternatives, helps you manage your treatment confidently.
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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.
Mounjaro Pen Details and Dose Titration
Mounjaro KwikPen specifications:
- Total volume per pen: 3 ml
- Total doses per pen: 4 (one injection per week)
- Injection device: Pre-filled auto-injection pen
- Storage: Refrigerated at 2–8°C before first use; room temperature (below 30°C) for up to 30 days after opening
The dose titration schedule typically runs for 16–20 weeks. You start at 2.5 mg weekly, then escalate by 2.5 mg increments every 4 weeks: 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg (maximum). Each dose is delivered as a full injection from the KwikPen; you cannot split doses. Individual needles are not visible—the pen uses a hidden needle system. Once you finish one 4-week pen, you'll move to the next pen (if continuing treatment).
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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.
Mounjaro vs Wegovy: Pen Volume and Mechanism
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injection pens licensed in the UK for weight management, but they differ in volume, mechanism, and dosing. Mounjaro's 3 ml pen delivers a dual GIP and GLP-1 receptor agonist—activating two hormone pathways. Wegovy uses a single GLP-1 mechanism. Clinical trials showed Mounjaro achieved average weight loss of 15–21% (SURMOUNT data), whilst Wegovy showed approximately 15% average loss (STEP data). Both require a clinical assessment to determine individual suitability. Your prescriber will decide which medication aligns with your health profile, eligibility, and preferences—neither is universally superior.
- 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.
How many times can I use one Mounjaro pen?
One Mounjaro KwikPen contains 4 pre-filled doses, enough for 4 weeks of once-weekly injections. You cannot reuse or refill a pen once it's empty. After 4 weeks, you'll need a new pen if continuing treatment. Always check the expiry date on your pen before use.
Can I adjust the volume or split a Mounjaro dose?
No. The KwikPen delivers a fixed, pre-set dose volume each time—you cannot manually adjust or split doses. The pen automatically delivers the correct amount for your prescribed dose strength (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg). If your prescriber changes your dose, you'll receive a new pen with a different dose strength.
What's the difference between Mounjaro and Wegovy pen volumes?
Mounjaro pens contain 3 ml total and deliver tirzepatide (dual GIP+GLP-1). Wegovy pens contain a different volume and deliver semaglutide (GLP-1 only). Both are once-weekly injections, but they use different mechanisms. Neither pen volume reflects clinical effectiveness—individual results vary based on dose, lifestyle, and personal factors.
How do I store my Mounjaro pen correctly?
Before first use, store your Mounjaro pen in a refrigerator at 2–8°C in its original carton. After first injection, you can keep it at room temperature (below 30°C) for up to 30 days. Never freeze it. Always check the solution is clear and colourless before injecting. Discard the pen 30 days after first use, even if medication remains.
Will my GP or prescriber calculate the right pen volume for me?
Yes. Your prescriber assesses your health, BMI, eligibility, and other factors to determine your starting dose and escalation schedule. Use our free GLP-1 weight loss calculator and BMI calculator to understand your baseline health metrics before your consultation. Your prescriber will issue prescriptions for the specific dose strengths you need at each stage of titration.
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.