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
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How to Take Mounjaro — The Complete UK Guide
Taking Mounjaro correctly is essential for safe, effective weight management. Mounjaro (tirzepatide) is a once-weekly injection administered using a KwikPen device. This guide covers the complete process: how to prepare and store your pen, proper injection technique, dose timing, and what happens after your first injection. Whether you're starting Mounjaro through NHS specialist services or private prescription, understanding the practical steps ensures you get the most from this dual GLP-1 and GIP receptor agonist medication. Always follow your prescriber's specific instructions, as individual protocols may vary slightly based on your health assessment.
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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.
Mounjaro Dosing and Injection Schedule
Mounjaro is supplied as a once-weekly injection in a pre-filled KwikPen. Each pen contains 4 doses, giving you a month's supply. Your prescriber will start you on 2.5 mg and typically increase the dose every 4 weeks, following this standard titration schedule:
- Weeks 1–4: 2.5 mg once weekly (starting dose)
- Weeks 5–8: 5 mg once weekly
- Weeks 9–12: 7.5 mg once weekly
- Weeks 13–16: 10 mg once weekly
- Weeks 17–20: 12.5 mg once weekly
- Week 21 onwards: 15 mg once weekly (maximum dose)
You must wait at least 4 weeks between dose increases. Each dose should be taken on the same day each week — many people choose Wednesday or Monday for easy remembering. Your prescriber may keep you on a lower dose if you're tolerating side effects well and achieving good results. Never skip doses or attempt to increase doses yourself without guidance from your prescriber.
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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.
Proper Injection Technique and Site Selection
Mounjaro is injected under the skin (subcutaneously) once weekly. Common injection sites include: the stomach (abdomen) — most common and easiest to self-inject; upper thigh — good alternative for self-injection; upper arm — requires assistance from another person. Rotate your injection site each week to prevent lipohypertrophy (thickened skin). Before injecting, remove your KwikPen from the fridge 15–30 minutes before use to allow it to warm slightly — this reduces injection discomfort. Clean the injection site with an alcohol wipe and let it dry. Pinch the skin gently, insert the needle at 90 degrees, press the injection button slowly, and hold for 3 seconds. Use our free GLP-1 weight loss calculator above to track your expected progress at different doses.
- 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.
How do I store my Mounjaro KwikPen?
Unopened pens must be stored in a refrigerator at 2–8°C in the original carton to protect from light. Do not freeze. After first use, you can store the pen at room temperature (below 30°C) for up to 30 days. Dispose of the pen after 30 days of use or on the expiry date, whichever comes first. Always keep away from direct heat and sunlight.
What if I miss a dose of Mounjaro?
If you remember within 4 days of your missed dose, inject as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on your normal scheduled day. Never take a double dose to make up for a missed injection. Maintain a minimum of 72 hours (3 days) between doses.
How long does a Mounjaro injection take?
The actual injection process takes only about 5–10 seconds. The entire preparation and injection routine typically takes 2–3 minutes once you're familiar with the process. It's a quick, simple procedure that most people can do easily at home. Some people find it helpful to set a weekly phone reminder for their injection day.
Can I inject Mounjaro while pregnant or breastfeeding?
No. Mounjaro is not recommended during pregnancy or whilst breastfeeding. If you become pregnant while taking Mounjaro, stop the medication and contact your GP or prescriber immediately. If you're planning to become pregnant, discuss this with your prescriber at least 3 months in advance, as you may need to switch to alternative weight management approaches during this time.
What happens if I inject Mounjaro incorrectly?
Minor injection technique errors (slightly wrong angle, shallow depth) usually won't prevent the medication from working, though you may have bruising or discomfort. If you didn't hear or feel the click confirming injection, the full dose may not have been delivered. Contact your prescriber or pharmacist for advice. Never re-inject if you're unsure — wait until your next scheduled dose. Your prescriber can demonstrate proper technique at your initial consultation.
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.