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
What Is The Shelf Life Of Mounjaro?
Mounjaro (tirzepatide) KwikPen devices have a shelf life of up to 3 years when unopened and stored correctly. Once you start using a pen, it remains stable for 30 days at room temperature or in the refrigerator. Understanding proper storage of your weight management injection is essential for maintaining effectiveness and safety. This guide covers shelf life, storage conditions, and how Mounjaro compares to Wegovy in terms of stability and storage requirements.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
Storage Requirements And Shelf Life Timeline
Unopened Mounjaro KwikPen:
- Shelf life: up to 3 years from manufacture date (check the expiry date on the box)
- Storage: refrigerate at 2°C to 8°C
- Keep in original carton to protect from light
- Do not freeze — if accidentally frozen, the pen cannot be used
- Do not remove from refrigerator until ready to use
After first injection:
- Remains stable for 30 days maximum
- Can be stored at room temperature (below 30°C) OR in the refrigerator — either is acceptable
- Keep the pen cap on when not in use
- Must be disposed of after 30 days, even if product remains in the pen
- Do not attempt to use beyond 30 days after first use
Each KwikPen contains 4 doses (4 weeks' supply) at your prescribed dose. If you use one pen per week, a single KwikPen lasts exactly 4 weeks before requiring disposal.
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.
Mounjaro Vs Wegovy: Storage And Stability Comparison
Both Mounjaro and Wegovy have similar storage profiles, though with slight differences. Wegovy (semaglutide) also requires refrigeration (2°C to 8°C) when unopened and remains stable for 30 days after first use — identical to Mounjaro. The main practical difference is dose frequency: Mounjaro uses the Mounjaro KwikPen (once weekly), whilst Wegovy uses an auto-injection pen (also once weekly). Both medications require UK prescription and clinical assessment before starting. Storage stability does not vary significantly between the two, so your choice is unlikely to be influenced by shelf life considerations. Your prescriber or pharmacist will advise on which medication is more suitable based on your individual health profile, eligibility, dose titration requirements, and response to treatment.
- 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 the 30-day mark if I haven't finished the pen?
No. Mounjaro must be disposed of 30 days after first use, regardless of how much remains in the pen. This is a safety requirement — the medication's stability cannot be assured beyond 30 days. Do not attempt to use an opened pen after this deadline.
What happens if I accidentally freeze my Mounjaro pen?
Do not use it. Frozen Mounjaro cannot be safely used. If your pen freezes, contact your prescriber or pharmacist for replacement. Keep pens in a standard refrigerator, not the freezer compartment, to prevent accidental freezing.
Can I store opened Mounjaro at room temperature instead of the fridge?
Yes. Once opened, Mounjaro is stable for 30 days at either room temperature (below 30°C) or in the refrigerator. Choose whichever is most convenient — both storage methods are equally acceptable and do not affect the medication's effectiveness.
How long does an unopened Mounjaro pen stay good for?
Up to 3 years from the manufacture date, as long as it remains refrigerated at 2°C to 8°C and stays in the original carton. Always check the expiry date printed on your KwikPen box. Do not use if the expiry date has passed.
Is Mounjaro shelf life different from Wegovy?
No. Both Mounjaro and Wegovy require the same storage conditions (2°C to 8°C when unopened, 30 days after opening). The main differences are mechanism (Mounjaro is dual GIP+GLP-1; Wegovy is GLP-1 only) and dose range — storage stability is essentially identical between both weight management 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.