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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Can I Use My Mounjaro Pen After 30 Days?
No — Mounjaro KwikPen must be discarded 30 days after first use, even if it contains remaining doses. This is a strict safety requirement set by the manufacturer and MHRA. Once you first inject from a Mounjaro pen, you have exactly 30 days to use that pen before it must be safely disposed of. This applies whether the pen is stored at room temperature or refrigerated. Understanding this storage rule is essential for anyone using tirzepatide, as using an expired pen could pose safety risks.
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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.
Mounjaro Storage Rules and the 30-Day Rule
Unopened Mounjaro pens can be stored in a refrigerator at 2–8°C until their printed expiry date, which is typically 2–3 years from manufacture. Once you use a pen for the first time, the 30-day clock starts — regardless of how many doses remain.
- After first use, store at room temperature (below 30°C) for up to 30 days
- After 30 days, the pen must be discarded safely in a sharps bin
- Do not attempt to refrigerate a pen after first use to extend its life
- Never use a Mounjaro pen beyond 30 days after opening, even if it appears to contain unused doses
This differs from some other injectable medications. The 30-day limit is a strict safety requirement — Mounjaro pens are not designed to maintain sterility or medication stability beyond this period once opened. Your GP or prescriber should provide sharps disposal guidance.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Mounjaro vs Wegovy Storage — Key Differences
Wegovy (semaglutide) has identical storage rules to Mounjaro in the UK — 30 days maximum after first use, stored at room temperature or refrigerated. Both are once-weekly GLP-1/GIP receptor agonist injections with the same manufacturer-mandated safety timeframe. Neither medication can be used beyond 30 days post-opening. The main differences between Mounjaro and Wegovy are the mechanism (tirzepatide is dual GIP+GLP-1, whilst Wegovy is GLP-1-only) and dose escalation schedules — but storage rules are equivalent. If you're considering either medication as part of your weight management journey, both require a UK prescription and clinical assessment, and both follow identical discard protocols.
- 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 a Mounjaro pen after 30 days?
Using a Mounjaro pen after 30 days is not recommended. The medication may lose potency, the solution may be contaminated, and sterility cannot be assured. Discard safely in a sharps bin and use a new pen for your next dose.
Can I refrigerate my Mounjaro pen after first use to extend the 30 days?
No. Once opened, Mounjaro must be stored at room temperature (below 30°C) for the full 30 days. Refrigerating an opened pen will not extend its usable life and may affect the medication.
What if I miss a dose and my pen is past 30 days?
Discard the expired pen safely and use a new one for your next scheduled injection. If your next injection is more than a week away, contact your prescriber for guidance on timing your next dose.
How should I dispose of my Mounjaro pen after 30 days?
Place the used pen in a sharps bin (a yellow puncture-proof container). Do not put it in household waste. Your GP surgery, pharmacy, or NHS walk-in centre will accept sharps bins for safe disposal.
Is the Mounjaro 30-day rule the same for all patients in the UK?
Yes. The 30-day rule applies to all patients using Mounjaro in the UK, regardless of dose or NHS vs private prescription. It's a strict manufacturer and MHRA requirement for safety and medication stability.
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.