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 your first Mounjaro KwikPen, you have a limited time to use it safely. Understanding the expiration date and storage rules is essential to ensure your tirzepatide injection remains safe and effective. This guide covers UK-specific storage guidance, expiry timelines, and what to do when your pen reaches its expiration date.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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
After your first Mounjaro injection, the opened KwikPen can be stored at room temperature (below 30°C) for up to 30 days. This is the key rule you need to remember. Once you have used the pen for the first time, you must dispose of it after 30 days, even if there is medication remaining inside.
- Keep the pen at room temperature during this 30-day window — do not refrigerate it once opened
- Protect it from heat and direct sunlight
- Keep the cap on between injections
- Do not freeze the pen at any stage
- Each KwikPen contains 4 doses (4 weeks' supply)
If you miss a dose during your 30-day window, you can still use the pen for the remaining doses within this timeframe. However, once 30 days have passed since first use, the pen must be safely discarded — even if medication remains.
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.
Before You Open — Unopened Pen Storage
Unopened Mounjaro KwikPens have a much longer shelf life. Store unopened pens in the refrigerator at 2°C to 8°C until you are ready to use them. Keep them in their original carton to protect from light. Unopened pens can be used until the expiry date printed on the packaging if refrigerated correctly. Check your pen's expiry date before opening — if an unopened pen has reached its expiration date, do not use it and ask your pharmacist how to dispose of it safely.
- 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.
What happens if I use Mounjaro after 30 days of opening?
Do not use your Mounjaro after 30 days of opening. The medication may no longer be safe or effective. Dispose of the pen following your pharmacy's guidance. If you have missed doses, contact your prescriber or pharmacist to reschedule your injections within the safe 30-day window.
Can I refrigerate my Mounjaro after opening it?
No. Once opened, Mounjaro must be stored at room temperature (below 30°C). Refrigerating an opened pen may damage the medication. Always keep unopened pens in the fridge, but switch to room temperature storage once you have used the pen for the first time.
How do I safely dispose of an expired or unused Mounjaro pen?
Do not throw your Mounjaro pen in the household bin. Take it to your local pharmacy, which will dispose of it safely as medical waste. Most UK pharmacies accept used or expired pens for free — ask your pharmacist or check the NHS website for your nearest disposal location.
If my KwikPen expires mid-treatment, what should I do?
Contact your prescriber or the pharmacy that dispensed your medication immediately. They can arrange a replacement pen to continue your treatment without interruption. Do not attempt to continue using an expired pen, as its safety and effectiveness cannot be assured.
Does Mounjaro expire faster in warm weather?
Yes, heat accelerates medication degradation. Keep your opened pen below 30°C and away from direct sunlight. If your home regularly exceeds 25°C, store the pen in the coolest available location. If exposed to temperatures above 30°C for extended periods, discard the pen and obtain a replacement from your pharmacist.
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.