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 Does Mounjaro Last Once Opened?
Once you open a Mounjaro KwikPen, it can be safely stored at room temperature (below 30°C) for up to 30 days. After 30 days, the pen must be disposed of, even if medication remains inside. Understanding proper storage and expiry timelines is essential for patients using tirzepatide to ensure safety and effectiveness. This UK health guide explains exactly how long your Mounjaro KwikPen lasts once opened and the key storage rules you need to follow.
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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 Timeline: Before and After Opening
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C and can be used until the expiry date printed on the carton. Never freeze your KwikPen — if frozen, it cannot be used and must be disposed of safely. Keep the pen in its original carton to protect it from light. Once you use your first injection from a pen, the 30-day countdown begins.
After your first injection, you can store the Mounjaro KwikPen at room temperature below 30°C for exactly 30 days. This gives you flexibility to carry your pen during the week and store it without refrigeration. After day 30, the pen must be discarded — do not attempt to use it even if it contains remaining doses. Each KwikPen contains 4 weeks' supply (four 2.5 mg doses or four of your current dose), so timing typically aligns with your weekly injection schedule.
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.
Practical Storage Tips and Disposal Guidance
Mark the date you first open your Mounjaro KwikPen on the carton so you know exactly when the 30-day window ends. Store it away from direct heat, sunlight, and extreme temperatures. If you're travelling, room temperature storage makes Mounjaro portable — ideal for holidays or work trips, though you should keep a cold pack if possible for journeys longer than a few hours.
Dispose of your used pens safely in a sharps bin or return them to a pharmacy for disposal — never put them in household waste. Your GP surgery or community pharmacy can provide a sharps bin free of charge. If you miss a dose during your 30-day window, you can still use the pen as long as 30 days have not elapsed since opening. Always consult your prescriber or pharmacist if you're unsure about storage or have damaged a pen.
- 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 refrigerate Mounjaro after opening it?
You can, but you don't need to. Once opened, Mounjaro can be stored safely at room temperature (below 30°C) for up to 30 days. Refrigeration won't extend this period, so room temperature storage is fine for convenience and portability.
What happens if Mounjaro freezes?
If your KwikPen freezes, do not use it — the medication will be damaged. Frozen Mounjaro must be disposed of safely. Always store unopened pens in a fridge (2–8°C), never in a freezer.
Can I use Mounjaro after 30 days of opening?
No. After 30 days from first use, the pen must be disposed of even if medication remains. Always note the opening date on your carton to track when the 30-day window ends.
How do I dispose of my used Mounjaro pen?
Return used pens to your local pharmacy or GP surgery — they have sharps bins for safe disposal. Never place pens in household waste. Most pharmacies accept sharps free of charge.
What if I forget to take my weekly Mounjaro dose within the 30-day window?
As long as 30 days haven't passed since opening the pen, you can take your injection whenever you remember. However, you should aim for your regular weekly schedule. Contact your prescriber if you're unsure about timing.
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.