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 Long Does the Mounjaro Pen Last?
Each Mounjaro KwikPen contains four weekly doses, meaning one pen lasts approximately four weeks if used as prescribed. However, storage conditions significantly affect how long your pen remains usable. Understanding proper storage is essential for UK adults using this prescription-only tirzepatide injection for weight management. This guide explains exactly how long your Mounjaro pen lasts, from the pharmacy to your last injection, and what you need to know about keeping it safe and effective throughout its lifespan.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Many Doses Are in One Mounjaro Pen?
Each Mounjaro KwikPen is a pre-filled injection device containing four doses. Since Mounjaro is a once-weekly injection, one pen supplies exactly four weeks of treatment. The pen is designed for convenience — you simply inject once per week, rotating your injection site between your stomach (abdomen), upper thigh, or upper arm. After four weeks, you will need a new prescription and a fresh pen from your pharmacy. During dose titration (when your GP or prescriber is gradually increasing your dose from 2.5mg to your maintenance level), you may use multiple pens at different dose strengths over several months as you escalate. This means the timeline varies — if you are at the starting 2.5mg dose, one pen lasts four weeks, but if your prescriber increases you after four weeks, your next pen will be a higher dose strength. Always check with your pharmacy about refill timing and ensure you have your next prescription ready before your current pen runs out.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Storage: How Long Your Mounjaro Pen Stays Safe to Use
Unopened pens can be stored in a refrigerator at 2–8°C (in the original carton) until the expiry date printed on the box — usually around 2–3 years from manufacture. Once you have opened and used your first injection from a pen, you can keep that same pen at room temperature (below 30°C) for up to 30 days. After 30 days of use at room temperature, you must discard the pen even if it still contains insulin. Never freeze Mounjaro — if your pen accidentally freezes, it cannot be used. Keep your pens away from direct sunlight and heat. UK pharmacies will include storage instructions with your prescription, and your pharmacist can answer specific questions about your pens. If you are unsure about the expiry date or storage status of your pen, contact your pharmacy or GP before using it.
- 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 weekly injections does one Mounjaro pen give me?
One Mounjaro KwikPen contains exactly four doses, which means it lasts four weeks if you inject once per week as prescribed.
Can I keep my Mounjaro pen in the fridge after I start using it?
No. Once you have used your first injection from a pen, you should store it at room temperature (below 30°C) for a maximum of 30 days. After opening, it cannot go back in the fridge.
What happens if my Mounjaro pen freezes?
If your pen accidentally freezes, you must not use it. Frozen Mounjaro is no longer safe or effective. Store pens in the fridge (not freezer) and away from ice packs.
How do I know when my Mounjaro pen has expired?
Check the expiry date on the original carton. Unopened pens can be stored in the fridge until this date. Once you open a pen, discard it after 30 days regardless of the printed expiry date.
What if I run out of Mounjaro before my refill is ready?
Contact your GP or prescriber as soon as possible. Do not skip doses or wait without medical guidance, as this can affect your weight loss progress and treatment stability.
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.