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 Go Off? Storage and Expiry Guide
Yes, Mounjaro does go off — like all medicines, tirzepatide has an expiry date and specific storage requirements. Your Mounjaro KwikPen comes with a printed expiry date on the packaging, and once opened, the pen can only be used safely for 30 days at room temperature. Understanding proper storage and expiry is essential for safe and effective use of this prescription-only weight management injection. This guide covers everything you need to know about keeping your Mounjaro safe and effective.
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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 Requirements and Shelf Life
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C and kept in their original carton to protect from light. When stored correctly at this temperature, your Mounjaro will remain safe to use until the expiry date printed on the box — typically around 2–3 years from manufacture.
Once you first use a pen, the storage rules change. You can keep it at room temperature below 30°C for up to 30 days, but you must throw it away after 30 days, even if there are remaining doses. This applies regardless of whether the pen is in the fridge or at room temperature — the 30-day countdown starts from first use.
- Never freeze Mounjaro — if it freezes, do not use it
- Keep pens away from direct heat and sunlight
- Always check the expiry date before using a new pen
- Do not use if the liquid looks cloudy or discoloured
Clinically BackedWeight Loss
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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.
What Happens If Mounjaro Expires or Spoils?
If your Mounjaro reaches its printed expiry date, it should not be used. Expired medication may be less effective or unsafe. Similarly, if a pen is left out of the fridge for longer than 30 days after first use, or if it has been frozen, the tirzepatide inside may degrade and lose potency.
Using expired or improperly stored Mounjaro could mean you do not receive the intended dose, making your weight management treatment less effective. Always dispose of expired or compromised pens safely — ask your pharmacy how to dispose of unused medicines, or use the NHS Medicines Disposal Service. If you have missed doses or are unsure about your pen's safety, contact your prescriber before your next scheduled injection.
- 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.
How long can I keep Mounjaro after opening the KwikPen?
Once you use a Mounjaro pen for the first time, you can keep it for up to 30 days at room temperature (below 30°C) or in the fridge. After 30 days, you must dispose of it, even if doses remain. The 30-day limit applies whether the pen is opened or not after first use.
Can I store Mounjaro in the fridge after I start using it?
Yes. After first use, Mounjaro can be stored in the fridge at 2–8°C for up to 30 days, or at room temperature below 30°C for up to 30 days. Either way, once 30 days have passed, dispose of the pen safely.
What should I do if my Mounjaro freezes?
Do not use frozen Mounjaro. If your KwikPen has been frozen, discard it and contact your prescriber for a replacement. Freezing damages the tirzepatide solution and makes it unsafe to use.
How do I know if my Mounjaro has expired?
Check the expiry date printed on the side of your KwikPen box and on the pen itself. The date is usually written as "EXP" followed by a month and year. Never use Mounjaro after this date has passed.
What if I forget to refrigerate an unopened Mounjaro pen?
If an unopened pen has been left at room temperature for a short time (a few hours), it is usually still safe. However, prolonged exposure to heat damages tirzepatide. Always return unopened pens to 2–8°C storage as soon as possible. If unsure, contact your pharmacist or prescriber.
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.