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
What Happens If Mounjaro Isn't Kept in the Fridge?
If your Mounjaro KwikPen has been left out of the fridge, the safety depends on how long and how warm. Unopened pens must stay refrigerated at 2–8°C, but once you've started using one, it can stay at room temperature (below 30°C) for up to 30 days. If your pen has been exposed to heat or left out for an extended period, it may no longer be safe to use — and you should contact your prescriber or pharmacy for guidance before injecting.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Temperature & Storage Requirements
Unopened Mounjaro pens must always be stored in a refrigerator between 2°C and 8°C. If an unopened pen freezes (below 0°C), it cannot be used — do not inject a frozen dose. Keep pens in their original carton to protect them from light.
After you start using a pen, it can be kept at room temperature (up to 30°C) for a maximum of 30 days. Once 30 days have passed since first use, discard the pen safely — do not use it beyond this window, even if there is medication remaining.
- Never expose pens to temperatures above 30°C
- Do not leave in direct sunlight or near heat sources
- If you travel, use an insulated cooling pouch with ice packs to maintain the correct temperature
- Keep a note of the date you first use each pen to track the 30-day window
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 To Do If Your Pen Is Left Out
If an unopened pen has been left at room temperature for a few hours, it is usually still safe — brief exposure to room temperature does not necessarily damage the medication. However, if your pen has been left out for many hours, overnight, or in a warm environment (above 30°C), contact your prescriber or pharmacy for guidance before using it.
If you are unsure whether your Mounjaro KwikPen is still safe, it is always better to ask your healthcare provider or pharmacist than to risk injecting potentially damaged medication. They can advise you on whether to use the pen or request a replacement. Keep all your pens stored correctly going forward to avoid this situation — use your fridge's dedicated medicine shelf (usually coldest) rather than the door.
- 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 Mounjaro be used if left out of the fridge overnight?
It depends on the temperature. If left at room temperature (below 30°C) overnight, it may still be safe, but contact your pharmacist or prescriber first. If it was in a warm environment (above 30°C) or for more than 48 hours, do not use it. Always check with your healthcare provider before injecting if you're unsure.
What temperature kills Mounjaro medication?
Freezing (below 0°C) permanently damages Mounjaro — frozen pens cannot be used. Exposure above 30°C for extended periods may also damage the medication, though brief exposure is usually safe. Heat, direct sunlight, and temperatures above room temperature should always be avoided.
How long can Mounjaro stay out of the fridge after I start using it?
Once you've used a Mounjaro KwikPen for the first time, it can stay at room temperature (below 30°C) for up to 30 days. After 30 days, you must discard it, even if medication remains. Mark the start date on each pen so you know when to discard it.
If I accidentally freeze my Mounjaro, is it ruined?
Yes. Frozen Mounjaro cannot be used safely — the medication is damaged and injecting it could be harmful. If your pen has frozen, contact your prescriber or pharmacy immediately. They can assess whether you need a replacement dose or can adjust your schedule.
What's the best way to store Mounjaro if I'm travelling?
Use a portable cooling pouch (insulin travel case) with ice packs to maintain temperatures between 2–8°C. If you're travelling for longer than 30 days with an opened pen, keep it refrigerated throughout. For shorter trips with unopened pens, a cooling pouch is safest. Check airline rules before flying with Mounjaro.
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.