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
Free tracker included with every download this week
How Long Can Mounjaro Stay In The Fridge?
Mounjaro (tirzepatide) storage is straightforward but important for maintaining effectiveness. Unopened Mounjaro KwikPen devices can stay in the fridge for their full shelf life when stored at 2–8°C, typically until the expiry date printed on the box. Once you've used a pen for the first time, the timeline changes — you have 30 days to use it at room temperature before it must be discarded. Proper storage protects your medication and ensures each weekly injection delivers the intended dose.
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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: Unopened vs Open Pens
Unopened Mounjaro pens can remain in your fridge indefinitely until the expiry date, provided they're stored correctly at 2–8°C. Keep them in the original carton to protect from light exposure, and never freeze them — if accidentally frozen, the pen must be discarded and cannot be used. Check the expiry date on your KwikPen box before each injection; expired medication should not be used even if it appears normal.
- Store unopened pens at 2–8°C (standard fridge temperature)
- Keep in original carton away from direct light
- Do not freeze under any circumstances
- Check expiry date before use
Opened/used Mounjaro pens switch to different storage rules. Once you've injected your first dose, you can store the pen at room temperature (below 30°C) for up to 30 days. After 30 days from first use, it must be disposed of — even if there are still doses remaining in the KwikPen. This 30-day window applies whether you store it in the fridge or at room temperature after opening.
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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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Mounjaro vs Wegovy: Storage Comparison
While both Mounjaro and Wegovy are once-weekly injectable medications for weight management, their storage differs slightly. Wegovy (semaglutide) also requires refrigeration at 2–8°C when unopened and can be stored at room temperature for 30 days after first use — the same as Mounjaro. However, tirzepatide (Mounjaro's active ingredient) and semaglutide have different mechanisms: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Both require a UK prescription and clinical assessment. Use our GLP-1 weight loss calculator to explore potential outcomes, and consult your prescriber about which medication suits your individual circumstances.
- 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 my Mounjaro in a fridge that freezes occasionally?
No. Freezing damages the medication irreversibly. If your fridge temperature dips below 0°C, the pen cannot be used. Choose a fridge section that maintains steady 2–8°C, away from the back wall where it's coldest. Ask your pharmacy for storage advice if you're unsure.
What happens if I leave my Mounjaro pen out of the fridge for more than 30 days?
After 30 days at room temperature (whether opened or unopened), Mounjaro should be discarded. The medication may lose potency or become unsafe. Keep track of the date you first remove it from the fridge and set a phone reminder for day 30.
Can I keep Mounjaro in a travel cool bag instead of the fridge?
Yes, temporarily. A cool bag with ice packs can maintain 2–8°C for short trips (up to a few hours). However, for regular storage, a proper fridge is essential. Always keep the pen in its original carton and avoid direct ice contact to prevent freezing.
Do I need to refrigerate Mounjaro between each weekly injection?
Yes. Even though you've opened the pen, it must be refrigerated at 2–8°C between doses if you're not using it immediately. Room temperature storage applies only if you're away from a fridge — you still have 30 days total from first use before discarding.
Is Mounjaro storage different from Wegovy or other weight loss injections?
Mounjaro and Wegovy have identical storage rules: refrigerate unopened pens, store opened pens at room temperature for up to 30 days. Other GLP-1 medications like Saxenda (liraglutide) may differ — always check the patient information leaflet or ask your pharmacist for the specific medication you're prescribed.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.