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 You Open The Fridge?
Once you open a Mounjaro KwikPen and start using it, you have exactly 30 days to use it before you must discard it — even if there's medication left inside. This applies whether you store it in the fridge or at room temperature. Unopened pens can be kept refrigerated at 2–8°C until their expiry date, but the moment you use the first dose, that 30-day countdown begins. Proper storage is essential to keep your tirzepatide safe and effective, so understanding these rules helps you get the most from your prescription and avoid waste.
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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 Rules For Mounjaro After First Use
After your first injection, you have 30 days to use your Mounjaro pen — this is the manufacturer's guidance and applies regardless of how you store it. You can keep it at room temperature (below 30°C) or in the fridge (2–8°C) during this 30-day window; both methods are acceptable. Once 30 days have passed since first use, you must dispose of the pen safely in a sharps bin, even if medication remains. Never freeze Mounjaro — if it freezes, it becomes unsafe and must be discarded. Store unopened pens in the refrigerator in their original carton to protect from light and heat. If you're on a dose titration schedule with Mounjaro (starting at 2.5mg and increasing weekly), each pen lasts four weeks — meaning your first 2.5mg pen covers weeks 1–4, your 5mg pen covers weeks 5–8, and so on. Keep a record of when you first open each pen to avoid confusion.
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.
Cost And Access Context For Mounjaro In The UK
Mounjaro costs vary significantly depending on whether you access it through the NHS or privately. NHS access is through specialist tier 3 weight management services — your GP can refer you, though availability and waiting times vary by region (ICB). Private prescription costs typically range from £129–£320 per month depending on your dose, with each KwikPen (containing four weeks of injections) being the standard unit. When budgeting, factor in that you'll need four pens per month at maintenance doses. Some private prescribing services offer payment plans. Always ensure any provider is GPhC-registered and that a UK-licensed prescriber assesses you before issuing a prescription. Your prescriber will discuss whether Mounjaro suits your health needs and circumstances.
- 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 my Mounjaro pen after 30 days if I keep it in the fridge?
No. The 30-day limit applies regardless of storage method — fridge or room temperature. Once you've used the first dose, you must discard the pen after 30 days. This rule is set by the manufacturer to ensure the medication remains safe and effective.
What happens if I freeze my Mounjaro by mistake?
Do not use it. Frozen Mounjaro becomes unsafe and must be disposed of in a sharps bin. Store it at 2–8°C in the fridge or below 30°C at room temperature, but never freeze it.
How much does a month's supply of Mounjaro cost privately in the UK?
Private Mounjaro typically costs £129–£320 per month depending on your dose. Each KwikPen (four weeks) is the standard unit, so you'll need approximately four pens monthly at maintenance doses. Costs vary by prescribing service and dose level.
Can I get Mounjaro on the NHS in the UK?
Yes, but access is through specialist tier 3 weight management services, not standard GP prescription. Ask your GP for a referral. NHS availability varies significantly by region, and waiting times can be lengthy in some areas.
What's the difference between Mounjaro and Wegovy for storage?
Both are once-weekly injections with similar storage rules: unopened pens refrigerated until expiry, and 30 days maximum after opening. Mounjaro is a dual GIP/GLP-1 agonist while Wegovy (semaglutide) is a GLP-1 agonist only — your prescriber will discuss which suits your health needs.
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.