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 If Mounjaro Is Left In Your Pen
If you have Mounjaro left in your KwikPen after your injection, you can safely continue using it for up to 30 days after first use. However, there are important storage rules to follow to keep your medication safe and effective. This guide explains what to do, how long you can safely keep an opened pen, and when you should discard it. Following these steps helps protect your tirzepatide injection and ensures it works as intended.
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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 used your Mounjaro KwikPen for the first time, you have a 30-day window to use the remaining doses. During this time, storage is flexible:
- Room temperature storage: Keep the pen at temperatures below 30°C. This is suitable for travel or daily use.
- Refrigerator storage: You can also store an opened pen in a fridge at 2–8°C if you prefer, though this is not required once first used.
- Do not freeze: Never place your pen in a freezer. Frozen Mounjaro should not be used.
- Protect from light: Keep the pen in its original carton to shield it from direct sunlight.
- After 30 days: Any remaining medication must be safely discarded, even if the pen appears to have doses left. The medication's effectiveness cannot be assured after 30 days.
Write the date you first used your pen on the carton so you remember when the 30-day period ends. Your GP or prescriber can advise on safe disposal methods—many UK pharmacies accept unused medicines through their free returns scheme.
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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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When To Discard Your Pen Safely
You must discard your Mounjaro pen immediately if:
- The liquid appears cloudy, discoloured, or contains particles (it should always be clear).
- The pen has been frozen, even if it now appears normal.
- You've had the opened pen for more than 30 days.
- The expiry date has passed on the original carton.
To dispose of your pen safely, place it in a sharps bin if you have one (available from your pharmacy or GP), or seal it in a rigid plastic container before putting it in household waste. Never put needles or pens directly in regular recycling or rubbish. Your local pharmacy can collect unused or expired medicine free of charge through the Medicines Take Back scheme. This is the safest and most environmentally responsible way to dispose of Mounjaro.
- 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 left in the pen after 30 days?
No. After 30 days from first use, you must discard any remaining medication, even if doses appear to remain. The medication's stability and effectiveness cannot be assured beyond 30 days. Always write the start date on your carton to track this clearly.
What temperature should I store an opened Mounjaro pen at?
An opened Mounjaro pen can be stored at room temperature (below 30°C) or in a refrigerator at 2–8°C. Both are acceptable. The key is to keep it away from extreme heat, freezing, and direct sunlight. If traveling, room temperature storage is convenient for up to 30 days.
What if my Mounjaro pen has been accidentally frozen?
Do not use a pen that has been frozen. Discard it immediately. Freezing can damage the medication, and using it may be unsafe or ineffective. Contact your prescriber if you need a replacement pen. Unopened pens should never be frozen—always refrigerate at 2–8°C until use.
How do I know if my Mounjaro is safe to use?
Before injecting, check that the liquid is clear and colourless. If it appears cloudy, discoloured, or has visible particles, do not use it—discard safely and contact your prescriber for a replacement. Always check the expiry date on the carton too.
Where can I safely dispose of my used Mounjaro pen in the UK?
Return it to your local pharmacy using the free Medicines Take Back scheme—no prescription needed. Alternatively, ask your GP for a sharps bin or sealed disposal container. Never put needles or pens in regular recycling or household waste. Most UK pharmacies accept unused or expired medicines at no cost.
Ready to take control of your weight loss journey?
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