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
Why You Can't Use Mounjaro After 30 Days
Once you've opened a Mounjaro KwikPen and started using it, you can only use that same pen for a maximum of 30 days. After this time, even if there's still medication left inside, the pen must be safely disposed of and not used further. This is an important safety rule set by the manufacturer and UK regulators to protect your health. Understanding why this rule exists and how to manage your Mounjaro storage correctly is essential for safe, effective treatment.
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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.
Why the 30-Day Limit Exists for Opened Mounjaro Pens
The 30-day limit on opened Mounjaro KwikPens is a safety and stability requirement, not simply a waste-prevention measure. Once a tirzepatide pen is first used, the rubber seal is broken and the medication inside begins to be exposed to air and temperature fluctuations, even though the pen is designed to minimise this. After 30 days, the pharmaceutical stability of the medication cannot be assured by the manufacturer, Eli Lilly. This means the active ingredient (tirzepatide) may have degraded or changed, potentially making it less effective or, in rare cases, unsafe to use. UK regulatory guidance through the MHRA and NHS protocols strictly requires that opened pens are discarded after 30 days, regardless of how much medication remains. Your prescriber and pharmacist will emphasise this to ensure you always receive medication at full potency and safety.
- Opened pens stored at room temperature (below 30°C) can be used for up to 30 days
- Temperature changes and air exposure affect medication stability over time
- Using expired or degraded tirzepatide may reduce effectiveness or pose unknown risks
- This rule applies to all Mounjaro doses (2.5 mg through 15 mg)
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.
Proper Mounjaro Storage and Pen Disposal in the UK
To stay safe with your Mounjaro treatment, follow these UK storage guidelines carefully. Unopened pens must be stored in a refrigerator at 2–8°C and can be kept until the expiry date shown on the carton. Once you've opened a pen and taken your first injection, you can store it at room temperature (below 30°C) for up to 30 days—but not longer. Mark the date you first used the pen on the carton so you know when the 30-day window closes. After 30 days, place the used pen in a sharps bin (a yellow needle-safe container) and dispose of it at a pharmacy or your GP surgery—never in household waste. Your pharmacist can advise on sharps disposal locations near you. Planning your Mounjaro doses carefully, especially during dose titration when you're increasing from 2.5 mg upwards, helps ensure you use each pen efficiently and minimise waste.
- 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 accidentally use Mounjaro after the 30-day mark?
If you've used a pen past 30 days, contact your GP or prescriber immediately. Don't panic—a single expired dose is unlikely to cause immediate harm, but your prescriber needs to know. They may advise monitoring for any unusual effects and will ensure your next dose is from a fresh pen. Always check the date you first opened the pen to avoid this.
Can I use Mounjaro if it's been out of the fridge for more than 30 days?
No. The 30-day rule applies whether the pen has been in the fridge or at room temperature. Once opened, the medication is exposed to air and temperature changes. After 30 days, discard the pen regardless of storage method. Unopened pens can stay in the fridge until their printed expiry date.
Does the 30-day limit apply to unused pens still in the box?
No. Unopened Mounjaro pens stored in the refrigerator at 2–8°C are safe to use until the expiry date printed on the carton. The 30-day limit only applies to pens you've already opened and started injecting from.
What should I do with a partially used Mounjaro pen after 30 days?
Place it in a yellow sharps bin immediately after the 30-day mark passes. Do not attempt to use any remaining medication. You can dispose of sharps bins at any NHS pharmacy or GP surgery free of charge. Your pharmacist can tell you the nearest sharps disposal point.
How do I plan my Mounjaro doses to avoid wasting medication?
Each Mounjaro KwikPen contains 4 weeks' worth of doses (four 1 mg injections at any dose level). Because you inject once weekly, each pen typically lasts exactly 4 weeks—well within the 30-day safety window. If your dose is being titrated upwards, your prescriber will adjust your prescription timing to ensure new pens arrive when needed, minimising overlap and waste.
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.