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 Store Mounjaro — UK Storage & Temperature Guide
Mounjaro (tirzepatide) is a prescription-only injectable medication supplied in pre-filled KwikPen devices. Correct storage is essential to keep your medication safe and effective. This guide covers exactly how to store Mounjaro before and after you start using it, plus important temperature rules, what to do if your pen freezes, and how to dispose of used pens safely in the UK.
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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.
Mounjaro Storage Before First Use
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C (standard fridge temperature). Keep the pen in its original carton to protect it from light. Do not freeze Mounjaro — if a pen is accidentally frozen, do not use it. Frozen medication cannot be safely used and should be disposed of. Check the expiry date on the carton; unopened pens can be used until the date printed on the box if stored correctly in the fridge.
- Store in the main body of the fridge, not the door (temperature fluctuates less)
- Keep in original packaging — this protects the pen from light damage
- Do not store near the back wall where it may freeze
- Check expiry date before use
- Never leave pens at room temperature before first use
If you need to travel with an unopened pen, use an insulated travel case with cooling packs to maintain the 2–8°C range. Ask your pharmacist or prescriber for advice on portable storage options if you're travelling abroad.
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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 Storage After First Use
After you inject your first dose, you can store the pen at room temperature (below 30°C) for up to 30 days. Write the date you opened the pen on the carton so you know when the 30-day window expires. Do not refrigerate the pen once it has been opened — room temperature storage is correct. Keep it away from direct heat and sunlight.
- Storage window: 30 days maximum after first injection
- Temperature: below 30°C (no need to refrigerate)
- Keep the pen in its protective case or original box
- Dispose of the pen on day 31, even if it still contains medication
- Never return an opened pen to the refrigerator
If you forget to use your Mounjaro pen within 30 days of opening, do not use it — dispose of it safely and contact your prescriber for a replacement.
- 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 store Mounjaro in the door of the fridge?
No. The fridge door experiences temperature fluctuations each time it opens and closes. Store unopened pens in the main body of the fridge at 2–8°C, away from the back wall where it might freeze. This keeps the medication at a stable, safe temperature.
What if my Mounjaro pen freezes by accident?
Do not use a frozen pen. Frozen medication is unsafe and cannot be used effectively. Dispose of it safely and contact your prescriber or pharmacist for a replacement. To prevent freezing, store pens away from the back of the fridge and never place them directly against cold walls.
How do I know when to throw away an opened Mounjaro pen?
Write the date you first used the pen on the carton. You must dispose of the pen exactly 30 days after that date, even if it still has medication left. Do not use the pen after 30 days, as the medication may no longer be safe or effective.
Can I keep Mounjaro at room temperature before I use it?
No. Unopened pens must be refrigerated at 2–8°C until you're ready to use them. Room temperature storage (below 30°C) is only for opened pens, and only for up to 30 days after the first injection.
What's the difference between unopened and opened Mounjaro storage?
Unopened pens must be refrigerated (2–8°C) until expiry. Once you inject your first dose, the pen can be stored at room temperature (below 30°C) for up to 30 days, then must be disposed of. This difference is important — do not refrigerate opened pens.
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