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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Do You Keep Mounjaro in the Fridge Once Opened?
Once you open a Mounjaro KwikPen and use your first injection, you do not need to keep it in the fridge — in fact, the storage requirements change. Your opened pen can be stored at room temperature (up to 30°C) for a maximum of 30 days. This guide covers the exact storage rules for opened Mounjaro pens in the UK, helping you keep your tirzepatide injections safe and effective 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.
Storage Rules for Opened Mounjaro Pens
Once opened, Mounjaro storage is straightforward:
- Store at room temperature — between 15°C and 30°C
- Keep away from heat and direct sunlight
- Do not refrigerate once the pen has been used
- Discard the pen exactly 30 days after first use — even if medication remains
- If you accidentally refrigerate an opened pen, it can still be used
This 30-day window applies regardless of dose or how many injections you've taken from the pen. Your prescriber and pharmacy should provide a clear date to discard the pen — write this on your KwikPen for reference. Mounjaro (tirzepatide) pens are designed for once-weekly injections, so your opened pen will typically last 4 weeks before the next pen arrives.
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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 Mounjaro — Different Storage Rules
Unopened Mounjaro pens have different storage requirements:
- Store in the refrigerator at 2°C to 8°C
- Keep in the original carton to protect from light
- Never freeze — if frozen, discard the pen
- Can be used until the expiry date if refrigerated correctly
Once you receive your next Mounjaro KwikPen, keep it in the fridge until you're ready to use it. After your first injection, switch to room temperature storage. If you're unsure about your current pen's storage status or have questions about dose titration across your Mounjaro vs Wegovy journey, ask your prescriber or pharmacist — they can advise on your specific NHS UK prescription or private prescription details.
- 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 refrigerate an opened Mounjaro pen?
If you accidentally put an opened Mounjaro pen back in the fridge, it can still be used. The medication won't be damaged. However, room temperature storage is recommended for opened pens. Always check the pen hasn't been frozen — if ice crystals are visible, discard it.
Can I travel with an opened Mounjaro pen?
Yes. An opened Mounjaro pen can be kept at room temperature during travel for up to 30 days from first use. Keep it away from extreme heat (over 30°C) and direct sunlight. Use a protective case or pen holder if possible. Never store in a hot car or direct sunlight.
How do I know when to throw away my opened Mounjaro pen?
Mark the date on your KwikPen when you first use it. Discard it exactly 30 days later, even if medication remains. Your pharmacy should also provide a disposal date. If you lose track, ask your GP or pharmacist for clarification on your specific prescription.
Can I store an opened Mounjaro pen in a medicine cabinet?
Yes, a medicine cabinet works well for opened pens if it stays below 30°C and away from direct sunlight. Avoid bathrooms (humidity fluctuates) or kitchens near heat sources. A bedroom drawer, cupboard, or cool corner of the kitchen is ideal.
What's the difference between unopened and opened Mounjaro storage?
Unopened pens must be refrigerated (2–8°C) in the original carton. Once you take your first injection, switch to room temperature (up to 30°C) for 30 days maximum. This dual storage system is standard for Mounjaro KwikPens across NHS and private prescriptions in the UK.
Ready to take control of your weight loss journey?
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