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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How Long Does Mounjaro Last Once Opened?
Mounjaro (tirzepatide) has specific storage rules depending on whether the KwikPen is opened or unopened. Once you start using a Mounjaro pen, it can be kept at room temperature for up to 30 days maximum, then must be discarded. Understanding these expiration dates is crucial for safe and effective use of this prescription weight loss injection. This guide explains Mounjaro storage rules in the UK context and compares them with Wegovy pen storage requirements.
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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 Storage and Expiration Timeline
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C and remain usable until the expiry date printed on the carton—typically up to 3 years from manufacture. Keep unopened pens in their original packaging to protect from light and never freeze them.
Once opened and first used, Mounjaro must be discarded after 30 days maximum. During this 30-day window, the pen can be stored either in the refrigerator or at room temperature below 30°C. You do not need to keep it refrigerated after opening, though refrigeration is an option if you prefer.
- Day 1 of first injection: pen can be used for up to 30 more days
- Day 30: discard the pen, even if medication remains inside
- After 30 days, tirzepatide degrades and loses effectiveness
- Never attempt to use a pen beyond 30 days after first use
Each Mounjaro KwikPen contains 4 weekly doses (4 weeks' supply), so most users will finish a pen before the 30-day expiration occurs. This differs from Wegovy (semaglutide), which also has a 30-day window once opened but uses smaller individual doses per injection.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Mounjaro vs Wegovy: Storage and Shelf Life Comparison
Both Mounjaro and Wegovy share the same 30-day post-opening expiration rule, but the medications work through different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist—the GIP component is unique to tirzepatide and distinguishes it from Wegovy (semaglutide), which activates only the GLP-1 pathway. In clinical trials (SURMOUNT vs STEP), Mounjaro showed slightly higher average weight loss (~15–21% body weight) compared to Wegovy (~15% body weight), though individual results vary significantly and depend on dose titration and lifestyle factors.
Storage temperatures: both medications tolerate room temperature storage after opening. Both require a valid UK prescription and must be assessed by a qualified prescriber before use. Your GP or licensed prescriber will determine which medication—Mounjaro, Wegovy, or another GLP-1 option—is suitable for your individual health profile and eligibility based on BMI and weight-related health conditions.
- 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 of opening the pen?
No. Once a Mounjaro KwikPen is first used, it must be discarded after 30 days, even if medication remains inside the pen. The tirzepatide degrades after this period and loses potency. Always check your pen's opening date and mark it clearly to track the 30-day window.
Does Mounjaro need to stay refrigerated after I open it?
No. After first use, Mounjaro can be stored at room temperature (below 30°C) or in the refrigerator—you have the choice. Many people find room temperature storage more convenient for the 4 weeks they're actively using the pen.
What's the difference between Mounjaro and Wegovy storage?
Both have identical 30-day post-opening expiration periods and can be stored at room temperature after opening. The key difference is mechanism: Mounjaro is a dual GIP+GLP-1 agonist while Wegovy is GLP-1 only. Your prescriber determines which suits your health profile and clinical needs.
My Mounjaro pen still has medication in it but it's been 28 days. Should I use it?
If it has been 28 days since first use, you can safely use the pen. The 30-day window allows for use up until day 30. After day 30, discard it immediately, even if doses remain.
How do I know when I opened my Mounjaro pen?
Write the opening date directly on the pen with a permanent marker when you first use it. This makes it easy to track and ensures you don't accidentally use it past the 30-day limit. Most people finish a 4-week pen before expiration anyway.
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