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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What Happens to Mounjaro After 30 Days in the Fridge?
If your Mounjaro KwikPen has been in the fridge for 30 days after first use, it must be disposed of and not used. Once opened, Mounjaro can only be stored at room temperature for up to 30 days before it expires and becomes unsafe. Understanding proper storage is crucial for both safety and effectiveness. Whether you're using Mounjaro (tirzepatide) or considering Wegovy (semaglutide), correct storage ensures your medication works as intended and you get accurate dosing for your weight management journey.
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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 Storage Rules and Wegovy Comparison
Mounjaro storage after opening: Once you first inject from your KwikPen, you have exactly 30 days to use the remaining doses. After 30 days at room temperature (below 30°C), the medication degrades and becomes unsafe. You must discard the pen, even if doses remain unused.
Before first use: Unopened pens stay in the refrigerator at 2–8°C and remain safe until the expiry date printed on the carton. Never freeze Mounjaro — frozen pens should not be used.
Wegovy (semaglutide) storage: Unlike Mounjaro's once-weekly KwikPen design, Wegovy follows similar rules: 30 days at room temperature after opening, or refrigerated unopened until expiry. Both medications require the same careful storage to maintain potency and ensure safe, effective dosing throughout your treatment journey.
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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.
Why Storage Matters: Mechanism and Dose Titration Differences
Proper storage is essential because both Mounjaro (dual GIP+GLP-1 agonist) and Wegovy (GLP-1 agonist only) are complex biological compounds that degrade when exposed to heat or improper conditions. Mounjaro's dose titration protocol — starting at 2.5 mg and escalating weekly to a maximum of 15 mg — relies on consistent medication strength at each dose. If a pen has been stored incorrectly, the dose you inject may be weaker than intended, potentially affecting weight loss results and dose escalation timing.
Clinical trial data (SURMOUNT for Mounjaro, STEP for Wegovy) established effectiveness based on properly stored medication. Your GP or prescriber will assess UK NICE eligibility and personalise your dosing schedule. Always follow storage instructions precisely to ensure you receive the full therapeutic benefit of your prescription.
- 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 use my Mounjaro KwikPen after it's been 30 days since I first injected?
No. Once opened, Mounjaro must be discarded after 30 days at room temperature, even if doses remain unused. Using it after 30 days risks reduced potency and safety. Always note the date you first use each pen and dispose of it after 30 days.
What's the difference between unopened Mounjaro storage and after-opening storage?
Unopened Mounjaro pens stay in the fridge at 2–8°C until the expiry date on the carton. Once you first inject, you switch to room temperature (below 30°C) for up to 30 days only. This 30-day room temperature window applies only after first use.
Does Wegovy have the same 30-day storage rule as Mounjaro?
Yes. Wegovy auto-injection pens also have a 30-day room temperature limit after opening. Both medications use the same storage principle: unopened pens refrigerated until expiry, opened pens used within 30 days at room temperature.
Why does Mounjaro expire after 30 days at room temperature?
Mounjaro is a biologic medication — a protein-based compound that degrades when exposed to warm temperatures. After 30 days, chemical breakdown may reduce potency, making dosing unreliable. Proper storage maintains the medication's strength and safety.
What should I do if I've stored Mounjaro incorrectly or it's been 31 days?
Do not use the pen. Dispose of it safely (your pharmacy can advise on disposal methods) and contact your prescriber for a replacement. Never attempt to use expired or improperly stored medication, as potency and safety cannot be assured.
Ready to take control of your weight loss journey?
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