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 Long Does Mounjaro Last Once Opened?
Once you open a Mounjaro KwikPen and take your first injection, the pen can be stored safely for up to 30 days — whether kept in the fridge or at room temperature below 30°C. Understanding proper storage after opening is essential for keeping your tirzepatide medication effective and safe. This guide covers everything UK users need to know about storing opened Mounjaro pens, when to dispose of them, and how to manage your prescription supply to avoid waste.
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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.
Storage Guidelines After Opening Your Mounjaro Pen
After your first injection, an opened Mounjaro pen can be kept at room temperature (below 30°C) for a maximum of 30 days. It does not need to stay in the fridge once opened, though you can keep it refrigerated if you prefer — both options are safe.
Keep the pen away from heat, direct sunlight, and extreme temperature changes. Do not expose it to temperatures above 30°C or it may be damaged. If your pen has been left in a hot car or above 30°C, it is safest to discard it and start with a fresh one.
Each Mounjaro KwikPen contains 4 weekly doses (4 weeks supply). Once opened, you should mark the date on the pen so you know when your 30-day window ends. After 30 days from first use, dispose of the pen safely even if medication remains inside.
- Room temperature (below 30°C): up to 30 days after opening
- Refrigerated (2–8°C): also safe, can extend usable period
- Mark the open date on your pen
- Discard after 30 days from first use
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Storage Before Opening and Safe Disposal
Unopened Mounjaro pens must be stored in a refrigerator at 2–8°C in their original carton until needed. Never freeze them — if frozen, they cannot be used. Unopened pens can be stored until the expiry date printed on the box if kept refrigerated continuously.
When your 30-day period ends after opening, or if the pen is damaged, dispose of it safely using a sharps bin if available. If you do not have a sharps bin at home, ask your pharmacy where to dispose of used pens — many pharmacies accept them. Do not put Mounjaro pens in household recycling or general waste. Our GLP-1 weight loss calculator can help you track your dose schedule, so you always know when to open a new pen and when disposal is due.
- 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 use Mounjaro after 30 days if I haven't used all 4 doses?
No. Once opened, Mounjaro must be discarded after 30 days, even if doses remain. This is because the medication's stability cannot be assured after 30 days, and using expired doses may be ineffective or unsafe. Plan your prescription refills to align with your injection schedule.
What if I forget to use my Mounjaro for several weeks — can I still use it?
If your opened pen is still within the 30-day window, it is safe to use as long as it has been stored correctly below 30°C. Mark the open date on your pen to track this. If it is beyond 30 days, discard it and start a new pen. Always contact your prescriber if you have missed doses.
Is it safe to store opened Mounjaro in the fridge instead of room temperature?
Yes, completely safe. You can store your opened Mounjaro pen in the refrigerator (2–8°C) for the full 30 days if you prefer. Refrigeration does not shorten the usable period — you still have 30 days either way. Choose whichever suits your routine best.
What happens if my Mounjaro pen gets too warm or is left in a hot place?
If exposed to temperatures above 30°C, the tirzepatide inside may be damaged and less effective. If you suspect your pen has overheated — for example, left in a hot car — it is safest to discard it and use a fresh pen. Contact your prescriber about replacing it if needed.
How do I safely dispose of my used Mounjaro pen?
Use a sharps bin if available (these are often free from pharmacies). If you do not have one, ask your GP surgery or local pharmacy about sharps disposal — many accept used pens and injectors. Never put needles in household waste or recycling. Your prescriber can advise on local disposal services. ```
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