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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Does Mounjaro Expire After 30 Days? Storage and Wegovy Comparison
Yes, Mounjaro (tirzepatide) expires 30 days after you first use a KwikPen, even if doses remain in the pen. This is an important storage rule that differs slightly from some other weight loss injections. Understanding how long your Mounjaro pen lasts after opening—and how this compares to alternatives like Wegovy—helps you plan your prescription refills and avoid wasting medication. This guide explains the official guidance from Eli Lilly and the UK MHRA, plus key differences with other GLP-1 medications available in the UK.
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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 vs Wegovy: Storage Timelines and Expiry Rules
Mounjaro storage timeline after first use:
- Unopened pens: store in a fridge (2–8°C) until the printed expiry date
- After first injection: can be stored at room temperature (below 30°C) for up to 30 days
- After 30 days: must be discarded, even if insulin remains in the pen
- Never re-refrigerate a pen once it has been at room temperature
Wegovy (semaglutide) storage timeline:
- Unopened pens: refrigerated until expiry date
- After first use: can be stored at room temperature (below 30°C) for up to 30 days
- After 30 days: must be discarded
Both Mounjaro and Wegovy share the same 30-day room temperature rule. This means if you start a pen on day one, you have exactly 30 days to use the remaining doses before it must be disposed of safely, regardless of how much medication is left.
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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.
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Why the 30-Day Expiry Rule Matters for UK Patients
The 30-day expiry after opening protects you from using degraded medication. Injectable medications like Mounjaro and Wegovy contain stabilisers that break down over time at room temperature, meaning the active ingredient may become less effective or unsafe after 30 days. This is why your prescriber or pharmacy will calculate your prescription schedule to align with the 30-day window.
If you're on a weekly injection schedule (both Mounjaro and Wegovy are once-weekly), a single KwikPen contains 4 weeks' worth of doses. Each pen should last exactly 4 weeks at room temperature. Plan your prescription refills to arrive before the 30-day mark to avoid gaps in your treatment and ensure you always have a fresh pen ready.
- 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 Mounjaro after 30 days if I store it in the fridge?
No. Once a Mounjaro pen has been used and stored at room temperature, it must be discarded after 30 days regardless of refrigeration. The 30-day rule applies from the first injection date. Do not attempt to re-chill or reuse a pen after the 30-day window has passed.
How do I know the 30-day expiry date on my Mounjaro pen?
Mark the opening date on the pen with a pen or marker as soon as you use it for your first injection. Count 30 days forward from that date. Your pharmacy may also provide a disposal reminder or label. You can use our free progress tracker to log injection dates and set reminders for pen changes.
Is Mounjaro storage different from Ozempic or Saxenda?
Mounjaro and Wegovy both follow the 30-day room temperature rule. Saxenda (liraglutide) is a daily injection and comes in a different pen format, but follows similar storage guidelines. Always check the patient information leaflet (PIL) for the specific medication you've been prescribed.
What if my Mounjaro prescription doesn't arrive in time for the 30-day mark?
Contact your prescriber or pharmacy immediately. Do not use an expired pen. If there will be a gap, your prescriber may need to adjust your treatment schedule or expedite your next prescription. Many online pharmacies offer next-day or two-day delivery to prevent gaps.
How do I safely dispose of a used Mounjaro pen after 30 days?
Place the used KwikPen in a sharps container or a designated needle disposal bin. Never put it in regular household waste or recycling. Most pharmacies and GP surgeries accept used insulin pens and syringes for safe disposal at no cost—ask when you collect your prescription.
Ready to take control of your weight loss journey?
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