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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Can Mounjaro Be Kept Out Of The Fridge?
Yes, Mounjaro can be kept out of the fridge, but only after you first use a KwikPen. Unopened pens must stay refrigerated at 2–8°C, but once you start using a pen, you can store it at room temperature (below 30°C) for up to 30 days. This UK storage guidance applies whether you're prescribed Mounjaro through NHS specialist services or via private prescription following a clinical assessment. Proper storage helps maintain the effectiveness of tirzepatide and ensures your weekly injections work as intended.
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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.
Storage Rules: Before and After First Use
Unopened Mounjaro pens must stay in the fridge:
- Storage temperature: 2°C to 8°C (standard fridge temperature)
- Keep in original carton to protect from light
- Can be used until the expiry date if kept refrigerated
- Never freeze — if a pen freezes, do not use it
- Transport: use a cool bag if travelling to avoid temperature fluctuations
After first injection, room temperature storage is allowed:
- Store at room temperature: below 30°C
- Can stay out of the fridge for up to 30 days from first use
- Keep away from direct sunlight and heat sources
- Discard pen 30 days after first use, even if medication remains
- Do not return to the fridge once removed
This timing works well for once-weekly Mounjaro injections — each KwikPen contains four weekly doses, so one pen lasts approximately 28 days, which aligns with the storage window.
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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.
Mounjaro vs Wegovy: Storage Differences
Wegovy (semaglutide) has similar storage rules to Mounjaro: unopened pens must be refrigerated at 2–8°C, and after first use can be kept at room temperature below 25°C for up to 30 days. The main difference is that Wegovy's room temperature limit is slightly lower (below 25°C rather than 30°C), so you need to be more cautious about warmth. Both are once-weekly injections in a similar KwikPen format. Your prescriber will explain storage requirements when you receive either medication. Use our GLP-1 weight loss calculator to understand how both medications work and which eligibility criteria apply to you based on your BMI and health profile.
- 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 out of the fridge overnight?
Leaving an unopened Mounjaro pen out overnight (below 25°C) should be safe, but prolonged exposure above 25°C may reduce effectiveness. Once opened, it's designed to be kept at room temperature for up to 30 days, so occasional short periods above 25°C are not a problem. If you're concerned, contact your prescriber or pharmacist for advice.
Can I put Mounjaro back in the fridge after opening it?
No — once you remove an opened Mounjaro KwikPen from the fridge, it should not be returned to the fridge. After your first injection, keep the pen at room temperature (below 30°C) for the remaining four-week storage period. This is because temperature fluctuations can affect the medication's stability.
How long is Mounjaro good for after I open the KwikPen?
After your first injection, you have 30 days to use the remaining doses in that KwikPen at room temperature. Each KwikPen contains four weekly doses, so one pen typically lasts 28 days — leaving a small safety margin. Always check the expiry date on your pen and discard it after 30 days, even if medication remains.
Is Mounjaro storage the same for NHS and private prescriptions?
Yes — storage requirements are identical regardless of whether you receive Mounjaro through NHS specialist tier 3 services or via a private prescription following clinical assessment. Both unopened pens require refrigeration at 2–8°C, and opened pens can be stored at room temperature below 30°C for up to 30 days.
What should I do if my Mounjaro pen gets too hot?
If an unopened pen is exposed to heat above 25°C for a short time, it may still be safe to use — check it hasn't been frozen or left in direct sunlight. If it has been very hot (above 30°C) for extended periods, contact your pharmacist or prescriber for advice. Never use a pen that has been frozen, and when in doubt, ask your healthcare provider.
Ready to take control of your weight loss journey?
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