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 a Tirzepatide Pen?
A tirzepatide pen is the branded delivery device for Mounjaro, a prescription-only weekly injection used for weight management in the UK. The pen is called a KwikPen and contains tirzepatide, a GLP-1 and GIP receptor agonist that works by helping regulate appetite and food intake. Each pen contains four weekly doses and is injected once per week into the stomach, thigh, or upper arm. Understanding how this pen works and how to use it is essential if you're considering or have been prescribed this medication.
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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.
How the Tirzepatide Pen (KwikPen) Works
The tirzepatide pen, also known as the Mounjaro KwikPen, is a pre-filled auto-injection device designed for ease of use at home. Each pen delivers a single dose of tirzepatide and can be used four times (once per week for four weeks) before it needs to be replaced. The pen contains tirzepatide solution that activates two natural hormone pathways—GIP and GLP-1—helping to reduce appetite, increase feelings of fullness, and support weight management alongside a reduced-calorie diet and increased physical activity.
Key features of the tirzepatide pen:
- Pre-filled with enough tirzepatide for four weekly doses
- Easy-to-use button mechanism—no dial or complicated setup
- Colour-coded by dose strength (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg)
- Injected once weekly into the stomach, thigh, or upper arm
- Requires refrigeration when unopened; can be kept at room temperature for up to 30 days after first use
- Prescription-only in the UK—available through NHS specialist weight management services or private online prescribing following clinical assessment
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Tirzepatide Dosing and Storage
Tirzepatide dosing starts at 2.5 mg and is gradually increased over time, with a minimum of four weeks on each dose before escalation. The maximum recommended dose is 15 mg once weekly. Proper storage is critical: unopened pens must be kept in a refrigerator at 2–8°C and protected from light in their original carton. Once you've used a pen for the first time, it can be stored at room temperature (below 30°C) for up to 30 days. Never freeze the pen, as this will damage the medication. Always check the expiry date before use, and dispose of the pen safely in a sharps bin after the 30-day window following first use, even if doses remain.
- 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.
Is the tirzepatide pen the same as Wegovy?
No. The tirzepatide pen (Mounjaro KwikPen) contains tirzepatide and is a dual GIP/GLP-1 receptor agonist. Wegovy contains semaglutide and is a GLP-1-only agonist. Both are prescription-only weekly injections licensed for weight management in the UK, but they use different mechanisms and come in different doses and delivery devices.
How do I use the tirzepatide pen?
The KwikPen has a simple button mechanism. After preparing your injection site, you place the pen tip against your skin and press the button to deliver the dose. Most people inject into the stomach, thigh, or upper arm and rotate injection sites weekly. Your GP or prescriber will provide detailed instructions before your first injection.
Can I get a tirzepatide pen on the NHS?
Tirzepatide (Mounjaro) is available through NHS specialist tier 3 weight management services for eligible adults. Access varies significantly by region. You'll need a referral from your GP and assessment by a specialist service. Many areas have long waiting lists, so private prescription is also available through registered UK online pharmacies.
What happens if I miss a tirzepatide injection?
If you miss a dose within 4 days of the scheduled time, take it as soon as you remember and continue your normal weekly schedule. If it's been more than 4 days, skip the missed dose and inject on your normal scheduled day. Never take a double dose, and ensure at least 72 hours between injections.
Does the tirzepatide pen require special training?
While the KwikPen is designed to be user-friendly, your prescriber or practice nurse should provide training on proper injection technique, site rotation, and storage before you start. Written instructions and support videos are also typically provided. Contact your prescriber if you're unsure about anything during use.
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.