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
Can You Use Your Mounjaro Pen After 30 Days?
No — you should not use your Mounjaro KwikPen after 30 days of first use. Once you've opened and started using a pen, the MHRA and manufacturer guidance states it must be disposed of after 30 days, even if it contains unused medication. This is a safety and stability rule, not just a recommendation. Below, we explain why, how to store your pen correctly, and what to do when the 30-day period expires.
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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 Limit
After opening a Mounjaro KwikPen, you have exactly 30 days to use it. The 30-day clock starts from the moment you first inject from that pen — not from the date you received it. After 30 days, the pen must be safely disposed of, regardless of how much medication remains inside.
This rule applies to pens stored at room temperature (below 30°C). The reason is that once opened and exposed to air, the medication's chemical stability cannot be assured beyond 30 days. Temperature fluctuations, exposure to light, and the repeated needle insertion all affect the drug's integrity.
Storage before first use: Unopened Mounjaro pens can be refrigerated at 2–8°C and used until the expiry date on the carton. But once you've used even a single dose, the 30-day countdown begins.
- Write the date you first use your pen on the carton as a reminder
- Set a phone alarm for day 28 if you're likely to forget
- Do not attempt to refrigerate a pen after it's been opened — this does not extend the 30-day window
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.
What Happens After 30 Days — Disposal and New Pen
After 30 days, even if your Mounjaro pen still has medication inside, it must be disposed of safely. Do not continue using the pen. The risk is that the medication may no longer work as intended, and there is a theoretical risk of bacterial contamination or reduced potency.
To dispose of your used or expired Mounjaro pen, take it to a pharmacy — most UK pharmacies accept used insulin pens and injectable medication pens for safe disposal. Your GP practice may also accept them. Never throw a Mounjaro pen directly into household waste or down the sink.
Your prescriber will issue a new pen prescription for your next dose, and your next 30-day period starts fresh. If you're finding it difficult to remember the 30-day rule, ask your prescriber or pharmacist for written storage guidance at each prescription.
- 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 refrigerate my Mounjaro pen after opening it to extend the 30-day window?
No. Once opened, your Mounjaro pen must be kept at room temperature (below 30°C) and used within 30 days, regardless of refrigeration. Refrigerating an opened pen does not extend the 30-day limit and may affect the medication. Always store opened pens at room temperature away from heat and direct sunlight.
How do I know when my 30 days is up?
Write the date you first inject from your pen on the carton immediately. Count forward 30 days from that date — that's your disposal deadline. Set a phone reminder or mark it on your calendar. If you miss the date, dispose of the pen safely at a pharmacy even if medication remains.
What if I've only used one dose but 30 days have passed?
Dispose of the pen at a pharmacy. The 30-day rule applies whether you've used one dose or all four doses. The clock starts from first use, not from when you received the pen. Do not attempt to use any remaining medication after day 30.
Where can I safely dispose of my used Mounjaro pen?
Take your used or expired Mounjaro pen to any UK pharmacy that accepts medicines disposal — most community pharmacies do. You can also contact your GP practice, as many provide needle and pen disposal services. Never dispose of pens in household waste or down the sink.
If my pen expires before 30 days, should I still dispose of it?
Yes. Always dispose of any Mounjaro pen by whichever date comes first: the expiry date on the original carton (if unopened) or 30 days after first use (if opened). Check both dates and use the earlier one as your disposal deadline.
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.