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 Does the Mounjaro Pen Work?
The Mounjaro KwikPen is a pre-filled, once-weekly injectable device that delivers tirzepatide—a dual GIP and GLP-1 receptor agonist medication. The pen works by releasing a precise dose of medication under your skin each week, helping to regulate appetite and support weight management alongside diet and physical activity. It's designed for ease of use at home and requires only a simple injection technique. Understanding how your Mounjaro pen functions helps you use it safely and confidently. This guide covers the mechanism, delivery system, and what to expect when using this prescription-only medication in the UK.
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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 Mounjaro KwikPen Delivers Medication
The Mounjaro KwikPen is a pre-filled, disposable injection device containing tirzepatide solution. Each pen holds enough medication for one week's dose, delivered as a single subcutaneous (under-the-skin) injection. The mechanism works through a spring-loaded needle that you activate by pressing the injection button on the pen—you don't need to manually insert the needle yourself.
The needle is very fine (29-gauge) and typically causes minimal discomfort. The pen automatically stops after the full dose is delivered, usually within a few seconds. The tirzepatide medication then absorbs into the fatty tissue beneath your skin, entering the bloodstream over time to activate GIP and GLP-1 receptors in your brain and digestive system.
- Needle length: 8 millimetres
- Needle gauge: 29G (very fine)
- Activation: Simple button press
- Duration: Approximately 5–10 seconds per injection
- Storage: Refrigerated until use (2–8°C)
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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.
Eligibility, Prescriptions, and Next Steps in the UK
In the UK, Mounjaro is prescription-only and available through two pathways. NHS access is via specialist tier 3 weight management services following a GP referral, though waiting times vary significantly by region and ICB availability. Private access is available through GPhC-registered online clinics following a clinical assessment by a UK-licensed prescriber.
UK eligibility criteria typically include a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. Ethnicity-adjusted thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults. Your prescriber will assess suitability and discuss how Mounjaro compares to Wegovy (semaglutide) or other options. Once prescribed, your pharmacist will provide training on pen use, storage, and injection technique.
- 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 Mounjaro pen self-injecting or does a healthcare professional do it?
The Mounjaro KwikPen is designed for self-injection at home. You activate the needle by pressing the injection button yourself. However, you can ask a healthcare professional or trusted person to help if needed. Training is provided by your pharmacist before your first use.
How often do I use the Mounjaro pen?
Once per week, on the same day each week. Each KwikPen contains one week's dose. You rotate injection sites (stomach, thigh, or upper arm) each week to avoid irritation.
What's the difference between Mounjaro and Wegovy pens?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist; Wegovy (semaglutide) is GLP-1 only. Mounjaro doses range from 2.5–15 mg weekly; Wegovy goes up to 2.4 mg. Both are once-weekly injections, though Mounjaro may produce slightly higher average weight loss in clinical trials.
Can I use the Mounjaro pen if I'm on other medications?
Mounjaro can interact with some medications, particularly oral contraceptives and diabetes medications. Always inform your prescriber of all medications you take. Oral contraceptives may need a barrier method for 4 weeks after starting or dose increases.
Where should I store my Mounjaro pens?
Store unopened pens in a refrigerator at 2–8°C in the original carton. After first use, pens can be kept at room temperature (below 30°C) for up to 30 days. Never freeze. Dispose of the pen 30 days after first 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.