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
Does Mounjaro Need To Be Refrigerated After Opening?
Yes, Mounjaro can be stored at room temperature after first use, but only for a limited time. Once you've opened a KwikPen, you can keep it below 30°C (away from heat and direct sunlight) for up to 30 days. After 30 days, the pen must be disposed of safely, whether it's been fully used or not. This guidance applies to all doses of tirzepatide, from your initial 2.5mg starting dose through to the maximum 15mg maintenance 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.
Storage Guidelines for Unopened and Opened Pens
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C. Keep them in the original carton to protect from light, and never freeze them — if a pen freezes, it cannot be used. You can safely use unopened pens until the printed expiry date as long as they remain refrigerated. This is the storage method used by pharmacies and recommended before you start your first dose.
After opening your first Mounjaro KwikPen, storage changes. You can keep the pen at room temperature (below 30°C) for up to 30 days, making it more convenient for daily travel or work. However, once 30 days have passed since first use, the pen must be discarded — even if there's still medication inside. This applies regardless of your tirzepatide dose or which week of your dose titration schedule you're on. Always keep the cap on when not injecting, and store away from heat sources and direct sunlight.
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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.
Practical Storage Tips and Disposal
Write the date you first open your Mounjaro KwikPen on the carton so you know when the 30-day period expires. If you're unsure whether a pen has been open for longer than 30 days, dispose of it safely rather than use it. Once you've used all four weekly doses from a KwikPen (or reached the 30-day mark), place the used pen in a sharps bin — your pharmacy can provide one free of charge, or you can request one from your GP practice. Never put Mounjaro pens in household waste or recycling bins. If you're considering Mounjaro as part of your weight management journey, our GLP-1 weight loss calculator can help you understand potential outcomes alongside these practical storage considerations.
- 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 again after taking it out?
No. Once you remove a Mounjaro pen from the refrigerator and begin using it, you should not put it back in the fridge. Keep it at room temperature (below 30°C) for the remaining 30 days. Repeated temperature changes can affect the medication's stability.
What if I accidentally freeze my unopened Mounjaro pen?
If an unopened pen freezes, it cannot be used safely. Contact your pharmacy or GP practice immediately — they can arrange a replacement. Never attempt to use a frozen Mounjaro pen, as the medication may be damaged.
Can I travel with my open Mounjaro pen?
Yes, you can carry your open pen while travelling as long as you keep it below 30°C and away from direct heat and sunlight. A small insulated pouch (not a freezer pack) can help on longer journeys, but refrigeration is not required once opened.
What's the safest way to dispose of my used Mounjaro pen?
Place used pens in a sharps bin, which your pharmacy provides free of charge. You can also ask your GP practice for a sharps bin. Never put Mounjaro pens in household waste or recycling, as they contain a needle and hazardous medication residue.
Does the 30-day limit apply to all Mounjaro doses?
Yes, the 30-day room temperature storage limit applies whether you're on 2.5mg, 5mg, 10mg, 15mg, or any other dose during your dose titration schedule. Always discard after 30 days from first opening, regardless of how much medication remains. ```
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.