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 You Use Mounjaro After 30 Days of Storage?
Once you open a Mounjaro KwikPen, you cannot use it after 30 days — this is a strict storage safety rule set by the manufacturer and UK regulatory authorities. After first use, your pen must be kept at room temperature (below 30°C) and disposed of safely after 30 days, even if there is still medication inside. Understanding these storage rules is essential for safe, effective treatment with tirzepatide, the active ingredient in Mounjaro. This guide explains UK storage requirements, what happens after the 30-day window, and practical tips for managing your doses.
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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 30-Day Storage Rule — What You Need to Know
The 30-day rule is non-negotiable. Once you first use your Mounjaro KwikPen, it must be disposed of exactly 30 days later, even if medication remains. This applies whether you store it in the fridge before opening or at room temperature after opening.
- Unopened pens: Store in a refrigerator at 2–8°C in the original carton until the expiry date printed on the box
- After first use: Can be kept at room temperature (below 30°C) for up to 30 days only
- Never freeze: If a pen freezes, do not use it — discard it safely
- Why 30 days? After opening, the insulin-like medication begins to degrade. Beyond 30 days, its effectiveness cannot be assured, and using expired or degraded Mounjaro may be ineffective or unsafe
Your GP or pharmacist can advise on safe needle disposal and provide a yellow sharps bin if needed. Never dispose of pens in regular household waste.
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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 Dose Scheduling — Plan Ahead to Avoid Waste
Planning your Mounjaro dosing schedule helps prevent waste and ensures continuous treatment. Each KwikPen contains four weekly doses, so one pen lasts approximately four weeks. Mark your calendar with your weekly injection day (always the same day each week) so you know exactly when to use each dose.
When ordering your next prescription, ensure it arrives before your current pen expires. If you miss a dose, you have up to four days after your scheduled injection day to take it — but do not use a pen that has passed its 30-day expiry from first opening. Discuss any gaps in treatment with your GP or prescriber, as missing doses may affect your weight loss progress. Our free GLP-1 weight loss calculator can help you track expected outcomes when on a consistent Mounjaro schedule.
- 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 try to use Mounjaro after 30 days?
Using Mounjaro after 30 days from first opening is not recommended. The medication may have degraded, making it less effective or potentially unsafe. Always dispose of your KwikPen after 30 days and do not attempt to use it beyond this window, even if medication appears to remain inside the pen.
Can I refrigerate my Mounjaro pen after I first use it?
No. Once you open and use your Mounjaro KwikPen, you must store it at room temperature (below 30°C) for up to 30 days only. Do not refrigerate an opened pen — this does not extend the 30-day window. Unopened pens should remain in the fridge until use.
How do I know when my 30 days is up?
Mark the date you first use your KwikPen on the pen itself with a pen or marker. Count 30 days from that date — that is your disposal deadline. Many people set a phone reminder for day 28 to ensure they remember to dispose of the pen safely before the deadline.
What if my new Mounjaro prescription hasn't arrived before my current pen expires?
Contact your prescriber or pharmacy immediately. Do not attempt to extend use of an expired pen. If there will be a gap, discuss this with your GP or weight management service — they may advise on how to manage the pause in treatment safely.
Is the 30-day rule the same for Wegovy or other GLP-1 medications?
Wegovy (semaglutide) has similar storage rules — 30 days at room temperature after first use, or refrigerated until use if unopened. Always check the patient information leaflet or ask your pharmacist, as exact storage requirements can vary slightly between tirzepatide and other GLP-1 or GIP/GLP-1 medications.
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.