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 Long Can Mounjaro Be Stored at Room Temperature?
If you're prescribed Mounjaro (tirzepatide), understanding proper storage is essential for maintaining the medication's effectiveness. Once your Mounjaro KwikPen has been opened, you can store it at room temperature (below 30°C) for up to 30 days. After this 30-day window, the pen must be discarded, even if it contains unused doses. Unopened pens should remain refrigerated at 2–8°C until you're ready to use them. Proper storage ensures your medication works safely and effectively throughout your treatment.
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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.
Room Temperature Storage Rules and Best Practice
Once opened, Mounjaro KwikPens can be stored at room temperature (below 30°C) for a maximum of 30 days. After opening, mark the date on your pen with a permanent marker so you know exactly when the 30-day window expires. This applies whether the pen is half-full or nearly empty—the clock starts from the moment you first inject yourself.
- Store opened pens away from direct sunlight and heat sources
- Keep away from moisture and high humidity
- Do not refrigerate after opening—this is important
- Do not freeze the pen at any point
- Discard after 30 days, even if doses remain
If you're switching between different Mounjaro doses during dose titration, each new pen follows the same 30-day rule once opened. When comparing tirzepatide access options, whether through NHS specialist services or private prescription, storage requirements remain identical across all approved prescriptions in the UK.
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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.
Unopened Pens and Proper Refrigeration
Unopened Mounjaro KwikPens must be stored in a refrigerator at 2–8°C (standard fridge temperature) in their original carton to protect from light. Do not freeze pens—if a pen freezes, it cannot be used and should be discarded. Unopened pens can be used until the expiry date printed on the box if kept properly refrigerated.
When you receive your prescription, whether from an NHS specialist weight management service or a private online prescriber, your pens arrive in a protective cool box. Transfer them to your home fridge immediately and keep them in the original carton. This ensures your Mounjaro remains stable and effective when you're ready to start or continue your dose titration schedule. If you're travelling, use an insulated travel case with cooling packs to maintain the correct temperature.
- 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.
What happens if I leave Mounjaro at room temperature for more than 30 days?
After 30 days at room temperature, the medication may no longer be safe or effective. You must discard the pen and obtain a new one from your prescriber. Never use Mounjaro beyond the 30-day room temperature limit, even if it looks normal.
Can I store Mounjaro in the bathroom or kitchen?
Avoid bathrooms due to humidity and temperature fluctuations. Kitchens are acceptable if the pen is stored in a cool cupboard away from direct sunlight and appliances that generate heat. Keep unopened pens in the main fridge, not the door where temperature varies.
Does room temperature storage affect Mounjaro's weight loss effectiveness?
Proper storage maintains clinical effectiveness shown in trials like SURMOUNT-1. Improper storage—freezing or exceeding 30 days at room temperature—can compromise the medication's action on GLP-1 receptors and may reduce the weight loss results you'd expect from treatment.
Is the 30-day room temperature rule different for different Mounjaro doses?
No. Whether you're on a 2.5 mg starting dose, 10 mg maintenance, or 15 mg maximum dose, the storage rule is identical: 30 days maximum at room temperature once opened. The dose doesn't change the stability window.
What's the difference between Mounjaro and Wegovy storage requirements?
Both are once-weekly prescription injections, but Wegovy (semaglutide) also follows similar storage rules: refrigerate unopened, room temperature (below 30°C) for up to 30 days once opened. Always check your specific medication's patient information leaflet for exact guidance.
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.