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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Mounjaro vs Wegovy Storage Conditions — UK Guidance
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they have different storage requirements. Understanding how to store your medication correctly is essential to maintain its effectiveness and safety. This guide compares the storage conditions for both medications and explains why they differ based on their chemical formulations and delivery systems.
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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 — Mounjaro vs Wegovy Compared
Mounjaro (tirzepatide) comes as a pre-filled KwikPen and has specific storage needs. Unopened pens must be kept in a refrigerator at 2°C to 8°C and should never be frozen — if frozen, the pen cannot be used. Keep the original carton in the fridge to protect from light. Once you start using a pen, it can be stored at room temperature (below 30°C) for up to 30 days before it must be discarded.
Wegovy (semaglutide) also comes as a pre-filled auto-injection pen with similar but slightly different guidance. Unopened pens require 2°C to 8°C refrigeration in the original carton. After first use, Wegovy pens can remain at room temperature (below 25°C) for up to 30 days. Like Mounjaro, frozen Wegovy must not be used.
Both medications should be kept away from heat and direct sunlight. Always check the expiry date on your pen — even if refrigerated correctly, expired medication should not be used. Using our GLP-1 weight loss calculator can help you track your treatment timeline and understand dosing schedules alongside storage planning.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Key Storage Differences and Clinical Considerations
The main practical difference is the room-temperature threshold: Mounjaro tolerates up to 30°C, while Wegovy is safer below 25°C — relevant if you live in a warm climate or travel during summer in the UK. Both require the same refrigeration range and both should be discarded 30 days after first use, regardless of how much medication remains.
Storage conditions matter because improper storage can damage the medication's structure, reducing its effectiveness. Neither medication should be used if it has been frozen, appears discoloured, or contains particles. Your prescriber will provide detailed storage leaflets with your prescription. If you're unsure about storage at any point, contact the pharmacy that dispensed your medication — they can advise on safe storage or replacement if needed.
- 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 or Wegovy at room temperature before first use?
No. Both medications must be kept refrigerated at 2°C to 8°C in their original cartons before first use. Only after you inject your first dose can the pen be stored at room temperature for up to 30 days.
What happens if Mounjaro or Wegovy freezes?
Frozen medication cannot be used safely. Freezing damages the tirzepatide or semaglutide structure, rendering it ineffective. If your pen has frozen, discard it and contact your pharmacy for a replacement.
How long can I keep Mounjaro or Wegovy after opening?
Both medications can be used for up to 30 days after first injection if stored correctly. After 30 days, discard the pen even if it contains unused doses. Mark your calendar when you first use a pen to track the deadline.
Can I travel with Mounjaro or Wegovy?
Yes, but with care. Keep unopened pens in a cool bag with ice packs during travel. After opening, the 30-day room-temperature window applies — Mounjaro up to 30°C, Wegovy up to 25°C. Carry your prescription documentation in case you're questioned at borders.
What should I do if my Mounjaro or Wegovy pen looks damaged or discoloured?
Do not use it. Contact your pharmacy or prescriber immediately. Discoloured solution, particles, or visible damage indicate the medication may have been stored improperly or is faulty and should be replaced to ensure safety and effectiveness.
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