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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Does Mounjaro Need to Be Refrigerated After Opening?
Yes, unopened Mounjaro pens must be refrigerated between 2–8°C, but after first use you can store them at room temperature (below 30°C) for up to 30 days. This is different from some other weight loss injections. Understanding correct storage is essential to keep your tirzepatide medication effective and safe. This guide explains Mounjaro storage requirements compared with Wegovy, and answers common questions about how to store your KwikPen.
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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.
Mounjaro vs Wegovy: Storage and Handling Differences
Mounjaro (tirzepatide) storage is straightforward once you understand the two phases. Unopened pens must stay in a refrigerator at 2–8°C in their original carton — they can be used until the printed expiry date if kept cold. After you inject your first dose, the used pen can be kept at room temperature below 30°C for exactly 30 days, then must be disposed of.
Wegovy (semaglutide) follows similar but slightly different rules. Unopened Wegovy pens are also refrigerated at 2–8°C until first use. After opening, Wegovy pens can also be stored at room temperature below 25°C for up to 30 days. The key difference is the slightly lower room temperature threshold — Wegovy prefers below 25°C, while Mounjaro tolerates up to 30°C.
- Both are once-weekly injection pens requiring refrigeration before use
- Both allow 30 days of room temperature storage after first injection
- Neither should be frozen — if frozen, discard and request a replacement
- Both should be kept away from direct sunlight and heat sources
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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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Practical Storage Tips and When to Replace Your Pen
Once you've used your Mounjaro KwikPen for the first time, mark the date clearly on the pen with a permanent marker. Count 30 days from that date — this is your discard deadline. If you forget the exact date, it's safer to dispose of the pen rather than risk using medication that may have degraded. You don't need to return it to a pharmacy; dispose of it in a sharps bin at home (available from most UK pharmacies for free) or take it to your local pharmacy for safe disposal.
Keep your pen in a cool cupboard or drawer away from windows once it's in use — a kitchen cupboard works well as long as it's not above the oven or radiator. Never leave Mounjaro in a hot car, near a radiator, or in direct sunlight. If you travel, consider a small insulated pouch for room temperature storage. Your GP or prescriber can issue a replacement pen if your current one is damaged or accidentally frozen.
- 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 fridge door after opening?
No — the fridge door is too warm. After opening, store your Mounjaro at room temperature below 30°C, ideally in a cool cupboard or drawer. Avoid the fridge door and any warm areas. Only unopened pens should be in the main fridge compartment.
What's the difference between Mounjaro and Wegovy storage requirements?
Both require refrigeration before use and allow 30 days at room temperature after opening. The main difference is room temperature tolerance: Mounjaro can be stored below 30°C, while Wegovy should be kept below 25°C. Both must be kept away from freezing and direct heat.
How do I know when to throw away my Mounjaro pen?
Mark the date of first use on your pen with a permanent marker. Discard it exactly 30 days later. If you're unsure of the date, dispose of the pen rather than risk using degraded medication. Use a sharps bin at home or take it to your local pharmacy.
Can I use Mounjaro if it's been accidentally frozen?
No — do not use frozen Mounjaro. If your pen has frozen, discard it safely and contact your prescriber for a replacement. Freezing damages the medication and makes it unsafe to inject.
Is tirzepatide storage the same as semaglutide storage?
Very similar but not identical. Both tirzepatide (Mounjaro) and semaglutide (Wegovy) require refrigeration before use and allow 30 days at room temperature after opening. The room temperature ceiling is slightly different — Mounjaro up to 30°C, Wegovy up to 25°C.
Ready to take control of your weight loss journey?
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