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
Do You Need To Keep Mounjaro In The Fridge?
Yes, unopened Mounjaro KwikPen devices must be stored in a refrigerator at 2°C to 8°C until first use. Once you've started using a pen, you can keep it at room temperature for up to 30 days. Proper storage is essential to maintain the medication's effectiveness. This guide explains exactly how to store your tirzepatide injection safely in the UK, what happens if it freezes, and when to dispose of your pen.
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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 Storage Requirements — Before and After First Use
Unopened pens must be refrigerated: Store Mounjaro KwikPen devices in a fridge at 2°C to 8°C until you use them for the first time. Keep them in their original carton to protect from light. Unopened pens can be used until the expiry date printed on the box, as long as they remain refrigerated.
After first use: Once you've injected your first dose, you can store the pen at room temperature below 30°C for up to 30 days. You don't need to refrigerate it anymore after opening. Write the date you started using it on the pen so you know when the 30-day window ends.
Never freeze Mounjaro: If a pen has been frozen, do not use it. Freezing damages the medication and makes it unsafe. If you accidentally freeze a pen, dispose of it and contact your prescriber for a replacement.
- Keep away from direct sunlight and heat sources
- Do not leave in a hot car or near radiators
- Dispose of any pen 30 days after first use, even if medication remains
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.
What If You Forget To Refrigerate Your Mounjaro?
If your unopened pen has been left out of the fridge for a short time (a few hours), it may still be safe to use — but this depends on how warm it got and for how long. If it's been at room temperature for more than a few hours before opening, contact your prescriber or pharmacist for advice rather than using it.
Once you've started using a pen, room temperature storage (below 30°C) is intentional and safe for up to 30 days, so occasional temperature fluctuations during this period are normal. However, if your in-use pen has been exposed to temperatures above 30°C for extended periods — for example, left in a hot car — it's safer to dispose of it and start a fresh pen.
When in doubt, always contact your prescriber or the pharmacy that supplied your Mounjaro. They can advise whether a pen is still safe to use.
- 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 you keep Mounjaro at room temperature before using it?
No. Unopened Mounjaro KwikPen devices must be kept in a refrigerator at 2°C to 8°C until first use. Room temperature storage before opening will damage the tirzepatide medication. Once you've injected your first dose, you can then keep the pen at room temperature below 30°C for the remaining 30 days.
What temperature should Mounjaro be stored at?
Unopened pens: 2°C to 8°C in a refrigerator. In-use pens: below 30°C at room temperature. Never expose Mounjaro to freezing temperatures or extreme heat. Keep away from direct sunlight and heat sources like radiators or car dashboards.
How long can you keep Mounjaro out of the fridge after opening?
After your first injection, an opened Mounjaro KwikPen can be stored at room temperature (below 30°C) for up to 30 days. After 30 days, dispose of the pen even if medication remains. Write the start date on your pen to track this.
What happens if Mounjaro freezes?
If a Mounjaro pen has frozen, do not use it. Freezing damages the tirzepatide and makes it unsafe. Dispose of the frozen pen and contact your prescriber or pharmacy for a replacement. Keep pens away from freezer sections and very cold environments.
Should you keep Mounjaro in the fridge when travelling in the UK?
If travelling, keep unopened pens in a cool bag or insulated travel case with ice packs to maintain 2–8°C if possible. Once you've started using a pen, you can safely carry it at room temperature for up to 30 days without needing refrigeration, making it easier to travel with.
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.