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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Why Does Mounjaro Need To Be Refrigerated?
Mounjaro (tirzepatide) is a biologically active injectable medication that requires refrigeration to preserve its stability and effectiveness. Like most injectable biopharmaceuticals, tirzepatide contains active peptide molecules that can degrade if exposed to heat or freezing temperatures. Storing your Mounjaro KwikPen correctly ensures you receive the full therapeutic benefit when you inject. Understanding proper storage is essential for anyone prescribed this medication 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.
Storage Requirements and Temperature Stability
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C until you start using them. This temperature range preserves the active tirzepatide and maintains the formulation's integrity. The pens should be kept in their original carton to protect them from light exposure, which can also degrade the medication. Never freeze Mounjaro—if a pen freezes, it becomes unusable and must be discarded.
- Refrigerated storage maintains stability until the expiry date on the carton
- Once you open and use a pen for the first time, you can store it at room temperature (below 30°C) for up to 30 days
- After 30 days, dispose of the pen even if medication remains
- Keep away from direct heat and sunlight during room temperature storage
Unlike some medications, Mounjaro cannot be reconstituted or transferred between pens. Proper storage extends its shelf life and ensures consistent dosing across all four weekly injections in each KwikPen.
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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 vs Wegovy: Storage and Stability Differences
Wegovy (semaglutide) has similar but slightly different storage requirements. Both medications are GLP-1-based injectables, though Mounjaro is a dual GIP and GLP-1 receptor agonist while Wegovy is GLP-1 only. Wegovy also requires refrigeration at 2°C to 8°C when unopened and can be kept at room temperature below 30°C for up to 30 days once started. Both medications share the same fundamental reason for temperature sensitivity: their active peptide molecules require stable, cool conditions to remain effective.
- Both require identical refrigeration temperatures before first use
- Both allow room temperature storage for 30 days after opening
- Both must be discarded if frozen or exposed to extreme heat
- Prescribers should provide clear storage instructions with your KwikPen
- 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 freezer if I'm not using it soon?
No. Freezing Mounjaro damages the tirzepatide formulation irreversibly. If a pen freezes, you must discard it and obtain a replacement from your pharmacy. Always keep unopened pens in the main refrigerator compartment at 2°C to 8°C, not in the freezer.
What should I do if my Mounjaro accidentally gets warm during storage?
If your pen has been at room temperature for more than 30 days, or exposed to temperatures above 30°C for an extended period, contact your pharmacy or prescriber. Do not use the pen—it may not be safe. You may be entitled to a replacement depending on your prescription coverage or NHS arrangement.
Why is Mounjaro more temperature-sensitive than some other injected medications?
Mounjaro contains tirzepatide, a complex peptide hormone analogue. Peptide molecules are inherently sensitive to temperature changes because heat causes the protein chains to denature or break down. This reduces effectiveness. Refrigeration slows molecular degradation and maintains the medication's therapeutic potency throughout its shelf life.
Can I travel with Mounjaro and keep it cool outside the UK?
Yes, but plan ahead. Use a cool bag or insulated travel case with cooling packs to maintain 2–8°C during transport. Never leave pens in direct sunlight or in a hot car. When flying, pack Mounjaro in your hand luggage (not checked baggage, which may get too cold). Always carry documentation confirming your prescription if traveling abroad.
Is Mounjaro storage different from Wegovy in any practical way?
Storage requirements are virtually identical—both need refrigeration at 2–8°C unopened, and room temperature storage below 30°C for up to 30 days after first use. The main difference is that Wegovy is a once-weekly injection like Mounjaro, but individual prescriber instructions may vary slightly, so always follow the guidance provided with your specific prescription.
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