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What to Do With Out of Date Mounjaro
If your Mounjaro KwikPen has passed its expiry date, you should not use it. Like all medicines, tirzepatide becomes less effective and potentially unsafe once expired. This guide explains how to identify an out of date pen, why expiry dates matter, and how to safely dispose of expired Mounjaro in the UK. Always check the expiry date on your KwikPen before each injection to ensure you're using a safe, effective dose.
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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.
How to Identify an Expired Mounjaro Pen
Every Mounjaro KwikPen has a clear expiry date printed on the side of the pen and on the carton. The date is usually printed as month and year (e.g., 03/2025). Check this date before every injection. If the date has passed, do not use the pen. Even if the liquid looks clear and normal, the active ingredient (tirzepatide) may have degraded and could be less effective or unsafe. Additionally, expired pens may have compromised storage integrity. If you're unsure about your pen's expiry date, contact your prescriber or pharmacy. Our free GLP-1 weight loss calculator can help you track your treatment progress, but only with unexpired, properly stored medication.
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.
Safe Storage and Expiry Rules for Mounjaro
Mounjaro KwikPens have different expiry timelines depending on storage. Unopened pens in the fridge (2–8°C) can be used until the printed expiry date. After first use, a pen can be stored at room temperature (below 30°C) for up to 30 days, even if the printed expiry date is later. This means you may need to dispose of a pen before its printed expiry date if you've been using it for a month. Never freeze a Mounjaro pen — if frozen, it must be discarded. Keep your KwikPen in its original carton to protect it from light and temperature fluctuations. Proper storage extends the life of your medication and helps ensure each dose is safe and effective.
- 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 after the expiry date?
No. Expired Mounjaro should not be used. The tirzepatide may have degraded, making it less effective or potentially unsafe. Always check the expiry date before injecting. If your pen has expired, contact your pharmacy for a replacement.
How long can I keep Mounjaro at room temperature after opening?
Up to 30 days after first use, even if the printed expiry date is later. After 30 days, discard the pen. This applies only if stored below 30°C away from direct sunlight and heat.
What happens if I accidentally freeze my Mounjaro pen?
If frozen, the pen should be discarded and not used. Freezing damages the medication structure. Always store unopened pens in the fridge (2–8°C) and opened pens at room temperature (below 30°C).
How do I dispose of an expired or unused Mounjaro pen in the UK?
Take it to your local pharmacy. Most UK pharmacies offer free medicine disposal services and will safely destroy the pen and needle. Do not throw it in household waste or flush it down the toilet.
Can I get a replacement if my Mounjaro pen expires?
Yes. Contact your prescriber or the pharmacy where you obtained it. If the pen expires due to storage issues on your part, you may need to pay for a replacement. If it expired in pharmacy storage, they may replace it free of charge.
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.