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 Keep Mounjaro In The Fridge Once Opened?
Yes, but with important conditions. Once you open your first Mounjaro KwikPen and take your first injection, you can store it at room temperature — but only for a limited time. Unopened pens must stay in the fridge. Understanding correct storage is essential for keeping your prescription tirzepatide safe and effective. This guide explains the exact storage rules for Mounjaro in the UK, whether your pen is open or unopened, and what to do if you've stored it incorrectly.
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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.
Mounjaro Storage Rules — Unopened vs Opened
Unopened Mounjaro pens: Must be stored in a refrigerator at 2°C to 8°C. Keep in the original carton to protect from light. Do not freeze — if frozen, the pen cannot be used. Unopened pens can be used until the expiry date printed on the carton if kept refrigerated at all times.
Once opened (first injection taken): Your Mounjaro KwikPen can be stored at room temperature below 30°C for up to 30 days. This means you don't need to keep it in the fridge between weekly injections during those 30 days. After 30 days from first use, discard the pen — even if medication remains inside.
- Room temperature storage is permitted from the date of first use
- Keep away from direct sunlight and heat sources
- Do not place in a freezer or near ice packs once opened
- Do not refrigerate the open pen unless you prefer to — room temperature is sufficient
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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've Refrigerated An Opened Mounjaro Pen?
If you've stored your open Mounjaro pen in the fridge, this is completely safe. Refrigeration will not damage the medication. Some people prefer to refrigerate their open pen anyway — this is fine and may even feel more familiar if you've used other injectable medications. The key point is that refrigeration is optional for opened pens, not required. However, once you've refrigerated it, keep it consistently refrigerated or at room temperature — do not move it back and forth between cold and warm environments repeatedly, as temperature fluctuations can affect the medication.
- 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 refrigerate my Mounjaro pen after opening it?
Yes, you can refrigerate an open Mounjaro pen if you prefer. It's optional — room temperature (below 30°C) is acceptable for up to 30 days after first use. Some people find refrigeration helpful for managing the pen between weekly doses. Either way is safe.
How long can Mounjaro stay at room temperature once opened?
Up to 30 days from the date of first injection. After 30 days, you must discard the pen, even if it contains unused doses. Mark the opening date on your pen to track this easily.
What temperature is considered "room temperature" for Mounjaro storage?
Room temperature means below 30°C. In UK homes, this typically includes kitchens, bedrooms, and bathrooms at normal indoor heating. Avoid direct sunlight, radiators, or windowsills where temperature may exceed 30°C.
What happens if Mounjaro freezes?
If an unopened or opened Mounjaro pen freezes, it cannot be used safely. Discard it and contact your prescriber for a replacement. Do not attempt to thaw and use a frozen pen — the medication will be damaged.
Can I travel with my open Mounjaro pen?
Yes. If travelling within the UK or abroad, you can carry an open pen at room temperature for up to 30 days from first use. For longer trips, bring unopened pens and keep them cool using an insulated travel case with ice packs — keep the temperature at 2–8°C.
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.