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
What Happens If You Use Mounjaro After 30 Days?
If you use a Mounjaro KwikPen more than 30 days after opening it, you risk using medication that may no longer be safe or effective. Once a Mounjaro pen is opened and stored at room temperature, it should only be used for up to 30 days. After this period, the medication can deteriorate, and the pen should be safely disposed of. Understanding proper Mounjaro storage is essential to ensure the tirzepatide dose you're taking is reliable and safe for your treatment journey.
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Calculate Your BMI Instantly
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 the 30-Day Limit
Unopened Mounjaro KwikPens must be stored in a refrigerator at 2°C to 8°C and can be used until the expiry date printed on the carton. However, once you open a pen and begin using it, the 30-day countdown starts immediately.
After opening, Mounjaro can be stored at room temperature (below 30°C) for a maximum of 30 days. This applies whether you use the pen once or multiple times during that period. After 30 days from first use, the medication should be discarded, even if there is still liquid remaining in the pen.
- Do not store opened pens in the refrigerator after first use — room temperature only
- Keep the pen away from direct heat and sunlight
- Record the date you first open each pen to track the 30-day limit
- Never attempt to use a pen beyond 30 days, even if it looks normal
If you're managing your tirzepatide dose schedule and tracking your progress, our GLP-1 weight loss calculator can help you plan your injection schedule alongside your prescription.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 the Risks Are and How to Dispose Properly
Using Mounjaro after 30 days carries several risks: the medication may have degraded and lost potency, meaning your dose may not be effective; chemical changes in the solution could potentially cause harm; and sterility cannot be assured once the pen has been open and exposed to room temperature for longer than the recommended period.
Proper disposal: Place the used or expired KwikPen in a sharps bin if you have one (ask your pharmacy), or follow your local authority's instructions for disposal. Never throw Mounjaro pens in household waste or down the toilet. If you're unsure, contact your pharmacy or GP surgery — they can advise on safe disposal in your area.
To avoid waste and ensure you always have fresh medication, ask your prescriber about timing your prescription refills so a new pen arrives before the previous one reaches its 30-day limit.
- 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 Mounjaro if it's been 31 days since I opened the pen?
No. Once a Mounjaro pen has been open for more than 30 days at room temperature, it should not be used. The medication may have degraded or become unsafe. Always discard pens that have exceeded the 30-day limit and request a fresh prescription from your prescriber.
How do I know when my Mounjaro pen opened date is?
Write the date you first open each KwikPen directly on the pen with a pen or marker. This helps you track the 30-day countdown accurately. Some patients use a calendar reminder or their phone to alert them when the pen reaches 28 days, so they order a replacement in time.
What if I only used my Mounjaro pen once in the 30 days?
The 30-day limit applies from the date of first opening, regardless of how many times you've used the pen. Even if you've only taken one dose, the pen must be discarded after 30 days at room temperature.
Can I refrigerate an opened Mounjaro pen to make it last longer?
No. Once opened, Mounjaro should only be stored at room temperature below 30°C. Do not attempt to refrigerate an opened pen — this may damage the medication. Store unopened pens in the fridge; opened pens at room temperature only.
What's the difference between Mounjaro and Wegovy in terms of storage?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) follow similar storage rules: unopened in the fridge, opened at room temperature for up to 30 days. Always check the patient information leaflet for your specific medication, as there may be minor variations depending on the formulation.
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.