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
How Long Does 1 Mounjaro Pen Last?
Each Mounjaro KwikPen contains enough medication for exactly 4 weekly injections, meaning one pen lasts for 4 weeks of treatment. The specific duration depends on your prescribed dose — whether you're starting at 2.5 mg, escalating through 5 mg, 7.5 mg, or 10 mg, or maintaining at the maximum 15 mg dose. All doses are administered once weekly, so one pen consistently covers a month-long supply. Understanding your supply timeline helps with prescription planning and avoiding gaps in treatment.
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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.
What's Inside One Mounjaro KwikPen?
A single Mounjaro KwikPen is a pre-filled injection device containing 4 doses of tirzepatide, with each dose administered once per week. The pen cannot be refilled — it is designed for single use across those 4 weeks, then disposed of safely.
- 4 weekly injections — one dose per week for 4 weeks
- Single dose per injection — delivered as a pre-set volume from the KwikPen
- Sterile, ready-to-use — no preparation or mixing required
- Colour-coded by dose — different pen colours indicate different strengths (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg)
This differs from some other weight loss injections — for example, Saxenda (liraglutide) is a daily injection, whereas Mounjaro's once-weekly schedule is more convenient for ongoing treatment. When comparing Mounjaro vs Wegovy, both are once-weekly, though they have different mechanisms: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only.
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.
Storage, Supply, and Prescription Planning
Before first use: Store unopened pens in the refrigerator at 2–8°C. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Always discard the pen after 30 days from first use, even if medication remains — do not store a partially used pen for longer than this.
Each pen lasts exactly 4 weeks on a consistent once-weekly schedule. Most prescriptions supply pens in batches covering 3 months (three pens = 12 weeks). If you're undergoing dose titration — gradually increasing from 2.5 mg to your target maintenance dose — you'll use a different coloured pen each month. Your prescriber or pharmacy will manage this schedule. If you require ongoing Mounjaro as a long-term treatment, ensure your UK prescription is renewed before your current supply ends to avoid gaps.
- 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 a Mounjaro pen after 4 weeks?
No. Each KwikPen is designed for exactly 4 weekly doses. After 4 weeks, dispose of the pen safely, even if it appears to contain medication. Once opened, never store a pen longer than 30 days. Your prescriber will issue a new pen for the following month.
What if I miss a dose — does my pen last longer?
Your pen still contains 4 doses. If you miss an injection, do not use a double dose the following week. Simply resume your normal weekly schedule. If you're more than 4 days late, skip that dose and inject on your next scheduled day. The pen remains a 4-week supply.
How do I know which dose colour pen I'm receiving?
Mounjaro KwikPens are colour-coded by strength: 2.5 mg (purple), 5 mg (blue), 7.5 mg (red), 10 mg (yellow), 12.5 mg (orange), and 15 mg (green). Your prescription will specify which colour you need. During dose titration, you'll step up to a new colour each month as directed by your prescriber.
Is Mounjaro available on the NHS, and how does the supply work?
Mounjaro access via the NHS is limited and varies by region. NICE (National Institute for Health and Care Excellence) has approved tirzepatide, but availability depends on your local Integrated Care Board and whether you're referred to specialist tier 3 or tier 4 weight management services. Private prescriptions are more readily available. Either way, you'll receive pens in scheduled batches.
What happens if I need to stop Mounjaro partway through a pen?
If your prescriber advises stopping treatment, dispose of the remaining pen safely at a pharmacy (via the sharps disposal or medicines disposal service). Do not attempt to store a partially used pen for future use. Your GP can advise on safe disposal and next steps.
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.