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 I Use Leftover Mounjaro From An Old Pen?
No, you should not use leftover Mounjaro from a partially used pen. Each Mounjaro KwikPen contains exactly four doses, designed to be used once weekly for four weeks. Once you finish a pen, it should be disposed of safely, even if it appears to have medication remaining. Using leftover or stored tirzepatide carries safety risks and is not recommended by UK prescribers or the MHRA.
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.
Why Mounjaro Pens Cannot Be Reused Safely
Mounjaro KwikPens are single-patient, single-use devices designed with precise dosing mechanisms. Once a pen has been used, several factors make reusing it unsafe:
- Dose accuracy: The injection mechanism may be compromised after four uses, making it impossible to assures an accurate 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg dose
- Needle integrity: The needle may have microscopic damage after the first use, increasing infection risk and injection site reactions
- Medication stability: Once refrigerated insulin-based medications are exposed to room temperature, their chemical stability degrades
- Regulatory compliance: The MHRA and your prescriber require adherence to the licensed dosing schedule — using leftover medication bypasses clinical oversight
- No sterility assures: After initial use, the pen is no longer sterile, even if capped and refrigerated
If your prescription allows tirzepatide dose titration (gradual escalation), your GP or prescriber will issue new pens at each dose level. Do not attempt to reuse pens to save costs — this is unsafe and not clinically endorsed.
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 Storage And Disposal Of Mounjaro Pens
Each Mounjaro pen should be stored correctly and disposed of appropriately:
- Unopened pens: Store at 2°C to 8°C in a refrigerator, away from direct sunlight, until the expiry date
- Opened pens: Can remain at room temperature (below 30°C) for up to 30 days after first use, then must be discarded
- Disposal: Place used pens in a sharp disposal container (yellow bin) — ask your GP practice, community pharmacy, or local waste disposal for guidance
- Never throw pens in household waste: This poses a sharps injury risk
- NHS cost: If you run out of Mounjaro before expected, contact your GP or tier 3 weight management service for a replacement prescription
If you have leftover pens you no longer need, speak to your prescriber or pharmacist about safe disposal rather than storing them indefinitely.
- 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 should I do if I have a partially used Mounjaro pen left over?
Do not attempt to use it. Dispose of it safely in a sharps bin at your GP practice or community pharmacy. Once a Mounjaro KwikPen has been used, the dosing mechanism and needle integrity cannot be assured for future doses, making reuse unsafe.
Can I refrigerate a used Mounjaro pen and use it later?
No. Once opened, a Mounjaro pen can be kept at room temperature for up to 30 days, but it should only be used for its four scheduled weekly doses. After that, it must be discarded regardless of whether it appears to contain medication.
How long does each Mounjaro pen last?
Each KwikPen contains four doses, designed for four weeks of once-weekly injections. After four weeks, even if medication remains, the pen should be disposed of safely.
If I've run out of Mounjaro pens, can I stretch one pen over more weeks?
No. Tirzepatide requires consistent weekly dosing as prescribed. If you run out, contact your GP or tier 3 weight management service immediately for a replacement prescription. Do not delay doses or stretch a pen.
What if I'm switching doses or stopping Mounjaro? What happens to old pens?
If your prescriber changes your tirzepatide dose (during titration) or if you stop treatment, unused pens should be stored correctly until they expire or disposed of safely. Ask your pharmacist for guidance on safe disposal at your local pharmacy. ```
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.