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: UK Instructions for Your KwikPen Injection
Mounjaro (tirzepatide) comes as a pre-filled KwikPen injection device that you use once every week. This guide explains how to prepare, inject, and store your Mounjaro safely in line with UK prescribing standards. Whether you've received your prescription through the NHS or privately, proper injection technique and storage are essential for getting the most from your treatment. Always follow the specific instructions provided with your prescription, and contact your prescriber or pharmacist if you're unsure about any step.
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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.
Understanding Mounjaro Doses and Starting Your Treatment
Mounjaro is prescribed as a dose titration schedule, meaning your dose increases gradually over several weeks. You start at 2.5 mg weekly, then escalate to 5 mg, 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg over at least 16–20 weeks. Each KwikPen contains four weekly doses. Your prescriber will provide a personalised schedule based on your weight, health, and how your body responds.
- Week 1–4: 2.5 mg injection once weekly
- Week 5–8: 5 mg injection once weekly
- Week 9+: Escalation continues if tolerated (minimum 4 weeks between dose increases)
- Maximum maintenance dose: 15 mg once weekly
Do not skip doses or increase your dose without guidance from your prescriber. Clinical trials showed that dose titration helps minimise side effects whilst allowing your body to adjust to the medication.
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.
Step-by-Step Injection Instructions and Storage Guidance
Before injecting, wash your hands and prepare your injection site (stomach, thigh, or upper arm). Remove the KwikPen from the fridge 15–30 minutes before use and inspect for any damage or cloudiness—do not use if damaged. Follow the visual instructions on your pen, dial the correct dose, pinch the skin, insert at 90 degrees, press the button, and count to 6 before withdrawing. Rotate your injection site each week to avoid irritation. Store unopened pens at 2–8°C in the fridge; after first use, you can keep the pen at room temperature below 30°C for up to 30 days. Never freeze your Mounjaro.
- 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 inject Mounjaro myself or do I need help from my GP?
Most people inject Mounjaro themselves once trained. Your pharmacist or nurse will provide demonstration and written instructions. If you cannot inject yourself (e.g. limited hand mobility), ask about having someone trained to help. You do not need a GP appointment for each injection—once per week at home is the standard approach.
What should I do if I miss a Mounjaro dose?
If you remember within 4 days of your missed dose, inject as soon as you recall it and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next scheduled injection on the normal day. Never take two doses in a row.
Does Mounjaro interact with oral contraceptive pills?
Yes. The UK Summary of Product Characteristics advises that Mounjaro may reduce absorption of oral contraceptives. If you take the contraceptive pill, use a barrier method (condom) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.
What are the common side effects when starting Mounjaro injections?
Nausea, diarrhoea, constipation, and stomach discomfort are very common, especially when starting or increasing your dose. These usually improve within days to weeks as your body adapts. Severe or persistent side effects require discussion with your prescriber. Use our GLP-1 weight loss calculator to estimate potential weight loss outcomes alongside these considerations.
Is Mounjaro available on the NHS in the UK?
NICE has approved tirzepatide (Mounjaro) for use within NHS specialist tier 3 weight management services for eligible adults. However, NHS availability and waiting times vary significantly by region. Many people access Mounjaro through private online prescribing with UK-licensed prescribers, which may be faster but involves private costs.
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.