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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Mounjaro Once Opened — UK Storage & Wegovy Comparison
Once you've opened your Mounjaro KwikPen, it can be stored at room temperature (below 30°C) for up to 30 days before the pen must be disposed of. This guide explains how to store your opened Mounjaro safely, how it compares to Wegovy in terms of storage and clinical effectiveness, and what you need to know about both medications to make an informed decision with your prescriber. We've included practical storage tips and a side-by-side comparison of the SURMOUNT and STEP clinical trial data for both tirzepatide and semaglutide.
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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.
Mounjaro Storage After Opening — Practical Guidance
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C and kept in their original carton to protect from light. Once you've used a pen for the first time, storage rules change significantly.
- Room temperature storage: Your opened Mounjaro KwikPen can be kept at room temperature (below 30°C) for up to 30 days after first use
- Never freeze: If your pen has been frozen, do not use it — freezing damages the medication
- Discard after 30 days: Even if you have doses remaining in the pen, it must be disposed of 30 days after first opening
- Keep away from heat and sunlight: Store in a cool, dark place away from direct sunlight
Each Mounjaro KwikPen contains 4 doses (4 weeks' supply), so your 30-day window gives you adequate time for your weekly injections. Your GP or prescriber will advise on how to dispose of used pens — usually through a pharmacy sharps disposal service.
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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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Mounjaro vs Wegovy — Mechanism, Trials, and Suitability
Mechanism: Mounjaro (tirzepatide) is a dual GIP + GLP-1 receptor agonist, whereas Wegovy (semaglutide) is a GLP-1-only agonist. This dual action in Mounjaro may help explain the weight loss differences observed in clinical trials. The SURMOUNT-1 trial showed Mounjaro at 15 mg resulted in approximately 20.9% average body weight loss over 72 weeks, while the STEP-1 trial showed Wegovy 2.4 mg achieved approximately 14.9% average weight loss over 68 weeks — though individual results vary significantly.
Both medications require a UK prescription and clinical assessment before prescribing. Your GP or licensed prescriber will determine which is more suitable based on your medical history, current medications, and personal circumstances. Neither medication is assured to work, and both are used alongside reduced-calorie diet and increased physical activity for best results.
- 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.
How long can I keep Mounjaro once I've opened it?
Once opened, your Mounjaro KwikPen can be stored at room temperature (below 30°C) for up to 30 days. After 30 days, the pen must be disposed of, even if you have unused doses remaining. Store it in a cool, dark place away from direct sunlight and heat.
What's the difference between Mounjaro and Wegovy storage?
Both are once-weekly injections, but Mounjaro storage after opening allows room temperature (below 30°C) for 30 days. Wegovy (semaglutide) follows similar storage rules. Both must be refrigerated unopened at 2–8°C and kept in their original cartons. Ask your prescriber for specific storage guidance for your medication.
Can I keep Mounjaro in the fridge after I've opened it?
Yes, you can. If you prefer to refrigerate your opened Mounjaro KwikPen, you can keep it in the fridge at 2–8°C for the full 30 days after first use. Many people choose this option for peace of mind, though room temperature storage is also safe if kept below 30°C.
Which achieved more weight loss — Mounjaro or Wegovy — in clinical trials?
In clinical trials, Mounjaro (tirzepatide) showed higher average weight loss — approximately 20.9% body weight at the 15 mg dose — compared to Wegovy (semaglutide) at approximately 14.9% at 2.4 mg. However, individual results vary significantly, and the choice between them depends on personal medical factors assessed by your prescriber.
Do I need an NHS prescription or can I get Mounjaro privately in the UK?
Mounjaro is available through both NHS specialist weight management services (tier 3) and private online prescribing services in the UK. NHS access requires a referral and assessment through your local specialist weight management service. Private access is available through GPhC-registered pharmacies following an online clinical assessment with a UK-licensed prescriber.
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.