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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Why Does Mounjaro Need To Be Refrigerated?
Mounjaro is a prescription-only injectable medication containing tirzepatide, a dual GIP and GLP-1 receptor agonist. Like many biologic medications, Mounjaro needs to be refrigerated to maintain its stability and effectiveness. The active ingredient in your Mounjaro KwikPen is sensitive to heat and temperature fluctuations, which can damage the medication and reduce its ability to work as intended. Understanding the correct storage requirements ensures your medication remains safe and effective throughout your treatment course.
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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 Store Unopened Mounjaro Pens
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C (36°F to 46°F) — this is the standard temperature for a domestic fridge. Store your KwikPens in the main body of the refrigerator, not on the door where temperature varies. Keep the pens in their original carton to protect them from light exposure, which can also damage the medication. Do not freeze Mounjaro — if a pen freezes, it cannot be used and must be discarded safely. Unopened pens can be stored this way until the expiry date printed on the carton, which is typically up to three years from manufacture. Check the expiry date before use. If you're prescribed a new Mounjaro dose or switching to a different tirzepatide product as part of your dose titration, follow the same refrigeration guidelines for all unopened pens.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Storage After First Use And Room Temperature Use
Once you've used an unopened Mounjaro KwikPen for the first time, the storage rules change. After first use, Mounjaro can be stored at room temperature below 30°C for up to 30 days. This allows flexibility if you're travelling or don't have refrigerator access immediately after your injection. However, after 30 days from first use, any remaining medication in that pen must be discarded — it cannot be refrigerated again for extended use. Keep the pen away from direct sunlight and heat sources. If you have spare unopened pens, continue storing them in the refrigerator until needed. This balance between refrigeration and room temperature storage makes Mounjaro more practical for UK users managing their medication at home or during travel.
- 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 freeze Mounjaro if I accidentally leave it in the freezer?
No. If Mounjaro freezes, it cannot be used and must be discarded. Freezing damages the tirzepatide molecule permanently. Always store in the main body of your fridge at 2–8°C, not in the freezer compartment. If you suspect a pen has frozen, do not attempt to use it.
What should I do if Mounjaro is left out of the fridge by accident?
If left unrefrigerated for a few hours, it's likely still safe to use — tirzepatide is stable for short periods at room temperature. However, if left out overnight or in direct sunlight for extended periods, discard it to be safe. Once you start using a pen, room temperature storage (below 30°C) is approved for up to 30 days.
Why is Mounjaro more expensive than some other weight loss injections if it needs refrigeration?
Refrigeration is not the primary cost driver. Mounjaro's cost reflects the dual GIP and GLP-1 mechanism, manufacturing complexity, and ongoing MHRA monitoring. Wegovy (semaglutide) also requires refrigeration, so storage needs don't significantly affect pricing between similar medications.
Can I store my Mounjaro KwikPen in a travel cooler or insulated bag?
Yes. If travelling, you can use an insulated cooler bag with ice packs to maintain 2–8°C during transport. Once you arrive, return the pen to a proper refrigerator. Do not rely solely on cooler bags for long-term storage — they are meant for short journeys only.
Do I need to throw away my Mounjaro if I'm waiting between doses?
No. Unopened pens can stay refrigerated until the expiry date on the carton. Once you start a pen and inject your weekly dose, that pen can be kept at room temperature for up to 30 days. Always rotate between unopened pens for your next dose if possible, keeping unused pens refrigerated.
Ready to take control of your weight loss journey?
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