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 to Do With Mounjaro Left in Your KwikPen
If you have leftover Mounjaro in your KwikPen, you'll want to know how long you can safely keep it. Once you've started using a pen, the medication has a limited lifespan — typically 30 days at room temperature or longer if refrigerated. UK guidance on storing tirzepatide is clear: keep unopened pens in the fridge, but after your first injection, there are specific rules you must follow. This guide explains exactly how to store and use your remaining Mounjaro safely, based on MHRA and NHS recommendations.
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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.
Storage Rules for Opened Mounjaro Pens
Once you've started using your Mounjaro KwikPen, storage conditions change. If the pen remains in the refrigerator at 2°C to 8°C after the first dose, you can continue using it beyond 30 days — up to the expiry date shown on the carton. However, if you've taken the pen out of the fridge, you have exactly 30 days to use it before you must discard it, even if there's medication left inside.
- Unopened pens: store at 2–8°C in original carton until expiry date
- After first use at room temperature: use within 30 days maximum
- After first use if kept refrigerated: use until expiry date
- Never refreeze a pen that has been left at room temperature
- If a pen has been frozen, do not use it — discard safely
Keep your pen away from heat, direct sunlight, and extreme temperatures. If you're unsure whether your pen is still safe to use, ask your GP or pharmacist for guidance.
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.
Using Remaining Doses Safely
If your Mounjaro pen still contains unused doses and is within the safe storage window, you can continue using it as normal. Each KwikPen contains four doses (one week's supply each), so if you've only used one or two doses, you have several weeks of medication remaining. Always check the expiry date on the carton and inspect the medication visually — if it looks cloudy, discoloured, or contains particles, do not use it and contact your pharmacist.
Your GP or prescriber can confirm whether continuing with your current pen is appropriate or if a new prescription is needed. If you're struggling with side effects or cost, speak to your healthcare provider — there may be alternative options available through NHS specialist services or different dosing strategies.
- 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 pen after 30 days if it's been kept in the fridge?
Yes. If your Mounjaro pen has been opened but kept refrigerated at 2–8°C, you can use it beyond 30 days up to the expiry date printed on the carton. However, once removed from the fridge, you have only 30 days before it must be discarded. Always check the carton expiry date before using.
What should I do if I've left my Mounjaro pen at room temperature for more than 30 days?
You must discard the pen safely — do not use it. Contact your pharmacist or GP for a replacement prescription. Never refreeze a pen that has been at room temperature, and never use medication that looks cloudy or discoloured.
How can I tell if my Mounjaro is still safe to use?
Check the expiry date on the carton and inspect the liquid visually. The medication should be clear and colourless. If it appears cloudy, discoloured, or contains particles, do not inject it. If in doubt, contact your pharmacist or GP before using.
If I've only used one dose from my Mounjaro pen, can I save the rest for later?
Yes, if stored correctly. Each KwikPen contains four weekly doses. If kept refrigerated after opening, you can use remaining doses up to the expiry date. If stored at room temperature, use all remaining doses within 30 days from the date you first opened the pen.
What's the best way to store my Mounjaro to keep it safe for longer?
Keep unopened pens in the fridge at 2–8°C in the original carton until needed. After first use, if you want to maximise storage time, keep the pen in the fridge rather than at room temperature. This extends usability from 30 days to the full expiry date.
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.