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 Mounjaro Last In The Fridge?
Mounjaro (tirzepatide) is a prescription-only weight management injection that requires proper storage to stay safe and effective. Unopened Mounjaro KwikPen devices must be kept in the fridge between 2°C and 8°C and can be stored until the expiry date printed on the carton. However, once you start using a pen, storage rules change significantly. Understanding correct storage is essential for maintaining the medication's effectiveness throughout your weight management journey.
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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.
Unopened Mounjaro Storage Rules
Unopened Mounjaro pens can be kept in the fridge for the full duration until their expiry date — typically 2 to 3 years from manufacture, depending on when the medication was produced. Keep unopened pens at 2°C to 8°C in the main body of your fridge, not in the door where temperatures fluctuate. Always store in the original carton to protect from light exposure. If your Mounjaro accidentally freezes, do not use it — discard the pen and contact your prescriber for a replacement. Never attempt to use frozen medication, as the drug's chemical structure may be damaged. Check the expiry date before each use, and keep a note of when you started a new pen so you know when it needs replacing.
Clinically BackedWeight Loss
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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.
After First Use — Room Temperature Storage
Once you use your first injection from a Mounjaro KwikPen, you can switch to room temperature storage (below 30°C) for up to 30 days. This gives you flexibility if you're travelling or prefer not to refrigerate after starting. However, you must dispose of the pen 30 days after first use, even if doses remain. Mark the start date on your pen with a permanent marker to avoid confusion. Keep the pen away from direct heat, sunlight, and humidity. If you forget to use a pen and it's been more than 30 days since first use, discard it safely and use a new one. Never refreeze a pen that's been at room temperature, and never leave it in a hot car or near radiators.
- 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 leave my unopened Mounjaro pen outside the fridge?
No. Unopened Mounjaro must always be refrigerated between 2°C and 8°C until first use. Storing it at room temperature will damage the medication and make it unsafe and ineffective. Always keep unopened pens in the main fridge compartment, not the freezer or door.
What do I do if my Mounjaro freezes?
Do not use frozen Mounjaro. Discard the pen immediately and contact your prescriber to arrange a replacement. Freezing damages the tirzepatide molecule, so the medication cannot be relied upon to work safely or effectively.
How do I know when to throw away my Mounjaro pen?
Check the expiry date on the original carton — unopened pens are valid until this date if kept refrigerated. Once you start using a pen, you must discard it 30 days later, regardless of how much medication remains. Write the start date on your pen as a reminder.
Can I travel with Mounjaro if I don't have a fridge?
Yes. Once your Mounjaro is in use, you can keep it at room temperature (below 30°C) for up to 30 days without refrigeration. This is ideal for travel. However, if the pen is unopened, you'll need to keep it cool — consider a small insulated travel case with ice packs.
Does Mounjaro need to stay in the original box after I start using it?
The original carton is only essential for unopened pens to protect from light. Once in use, you can store the pen elsewhere as long as it's kept between 2°C and 8°C (refrigerated) or below 30°C (room temperature), and not exposed to direct sunlight or heat.
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.