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 Mounjaro Starter Pack?
A Mounjaro starter pack contains everything you need to begin tirzepatide treatment for weight management in the UK. It includes four pre-filled KwikPen devices, each containing a single 2.5 mg dose — your starting injection for the first four weeks. Understanding what's included, how to use it, and what to expect in those critical first weeks helps you start confidently and safely.
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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 Included in Your Mounjaro Starter Pack
Your Mounjaro starter pack contains four pre-filled KwikPen injection devices, each with a 2.5 mg dose of tirzepatide. Each pen delivers exactly one weekly injection, giving you a four-week supply. The pack also includes an information leaflet with injection instructions, storage guidance, and details about side effects. You'll receive storage and safety information explaining that unopened pens must be kept in a refrigerator at 2–8°C, and that each pen can be stored at room temperature (below 30°C) for up to 30 days after first use.
- Four pre-filled KwikPen devices (2.5 mg each)
- Patient information leaflet (PIL)
- Storage and safety guidance
- Instructions for use and injection sites
- Prescriber contact details for follow-up support
The 2.5 mg dose is specifically designed as a starting dose only — your prescriber will guide you through dose escalation over the following weeks based on your response and any side effects.
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.
Getting Started: First Steps and What to Expect
Before your first injection, check that all pens have been stored correctly and haven't been frozen. Choose your injection site — the stomach (abdomen), upper thigh, or upper arm — and rotate a different site each week. Your first injection may cause mild nausea or stomach discomfort; this is very common and typically improves within a few days as your body adjusts. Keep a progress tracker to record any side effects, appetite changes, or mood shifts during your first four weeks. Contact your prescriber if side effects are severe or persistent. You'll need to schedule a follow-up assessment at week four to discuss your progress and plan your dose increase to 5 mg.
- 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.
Do I need a prescription for a Mounjaro starter pack?
Yes. Mounjaro (tirzepatide) is prescription-only in the UK and requires a clinical assessment from a licensed prescriber before the starter pack is dispensed. Your prescriber will confirm you're eligible based on your BMI and health circumstances.
How long does a starter pack last?
A Mounjaro starter pack contains four 2.5 mg pens, each for one weekly injection. One starter pack lasts exactly four weeks. After week four, your prescriber will typically increase your dose to 5 mg for the next phase of treatment.
Can I use the MyWeightCheck BMI calculator to check if I'm eligible for Mounjaro?
Our free BMI calculator helps you understand your personal BMI and how it compares to NHS thresholds. However, eligibility for Mounjaro requires a full clinical assessment by a licensed prescriber, who will consider your overall health, medical history, and any weight-related conditions.
What if I miss my first injection or delay it?
If you delay your first injection by a few days, simply take it as soon as remembered and continue your normal weekly schedule thereafter. Avoid taking two injections close together. Contact your prescriber if you're unsure about timing or have concerns about missed doses.
Is tirzepatide the same as semaglutide (Wegovy)?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are licensed for weight management in the UK, but they work slightly differently. Your prescriber will recommend the most suitable option based on your individual circumstances.
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.