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 to Use Mounjaro in the UK
Mounjaro (tirzepatide) is a prescription-only weight loss injection that you administer yourself once weekly at home. This guide explains how to use your KwikPen device safely, what dosing schedule to expect, where to inject, how to store your medication, and what happens if you miss a dose. Whether you're starting Mounjaro through NHS specialist services or private prescription, understanding proper injection technique and storage is essential for safety and effectiveness. Always follow your prescriber's instructions, and contact your GP or prescriber if you have questions about your specific treatment plan.
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.
Getting Started: Doses, Schedule, and What to Expect
Mounjaro comes as a pre-filled KwikPen device, with each pen containing 4 weekly doses. Your prescriber will start you on 2.5 mg for the first four weeks — this is an adjustment dose to help your body adapt. Every four weeks, the dose may increase by 2.5 mg (to 5 mg, then 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg as the maximum). Each dose is injected once per week on the same day each week. The tirzepatide dose titration is gradual because it helps minimise side effects like nausea and stomach upset.
- Starting dose: 2.5 mg weekly for 4 weeks
- Typical escalation: Increase every 4 weeks if well tolerated
- Maximum dose: 15 mg weekly
- Frequency: Once per week, any day you choose (but keep it consistent)
You can try our free GLP-1 weight loss calculator to see estimated outcomes at different doses based on clinical trial data — though individual results vary significantly.
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.
Injection Technique, Storage, and Practical Tips
Inject Mounjaro into your stomach (abdomen), upper thigh, or upper arm — rotate injection sites each week to prevent irritation. Wash your hands, let the pen warm to room temperature for 30 minutes before use, and inject at a 90-degree angle into a pinch of skin. Your prescriber or pharmacist should show you the correct technique before your first injection.
Storage: Keep unopened pens in the fridge at 2–8°C. After first use, store at room temperature (below 30°C) for up to 30 days. Never freeze. Missed dose? If within 4 days, inject as soon as you remember and resume your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day — never double-dose. Use the weight loss injection tracker or our free progress tracker to log your injections and monitor your weight loss journey.
- 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 switch between Mounjaro and Wegovy?
Both are prescription-only weight loss injections, but they work differently — Mounjaro is a dual GIP/GLP-1 agonist, while Wegovy is GLP-1 only. Any switch between medications must be managed by your prescriber; there is no standard conversion chart. Never switch without medical guidance.
What if I feel sick after my Mounjaro injection?
Nausea is common, especially in the first few weeks or during dose increases. Eat small, frequent meals, avoid fatty foods, stay hydrated, and speak to your prescriber if it's severe. Side effects often improve over time as your body adapts.
How long does Mounjaro take to work?
You may notice reduced appetite within days, but weight loss typically becomes measurable after 2–4 weeks. Clinical trials showed significant results over 72 weeks. Combined with a reduced-calorie diet and increased physical activity, results vary by individual.
Do I need a prescription for Mounjaro in the UK?
Yes, Mounjaro is prescription-only. Access is through NHS specialist tier 3 weight management services (with potential waiting lists) or through private online prescribing following a clinical assessment. Your eligibility depends on BMI and health factors.
Can I use our BMI calculator to check if I'm eligible for Mounjaro?
Yes — use the free BMI calculator on this page to find your current BMI. UK adults with BMI 30 or above (or 27+ with weight-related health conditions) may be eligible, though a prescriber must assess you individually for suitability.
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.