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 For Weight Loss
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK. If your GP or prescriber has assessed you as eligible, understanding the correct injection technique, dose schedule, and storage is essential for safe and effective use. This guide explains how to take Mounjaro step-by-step, from your first 2.5 mg starter dose through to maintenance, alongside practical storage and injection site advice. Always follow your prescriber's instructions and read the patient information leaflet that comes with your KwikPen.
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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 To Inject Mounjaro Correctly
Mounjaro comes as a pre-filled KwikPen device containing four weeks' worth of doses. Each injection is given once weekly, always on the same day of the week. You can inject at any time of day, with or without food — timing doesn't affect how well it works. The injection is given under the skin (subcutaneously) into three possible sites:
- Stomach (abdomen) — most common and easiest site
- Upper thigh — also commonly used
- Upper arm — requires another person to help
Rotate your injection site each week to prevent skin irritation or lumps. Clean the area with an alcohol wipe, pinch the skin, insert the needle at a 90-degree angle, and hold for 3 seconds before removing. Your prescriber or pharmacist will demonstrate the technique before your first dose. If you feel uncertain, ask for a demonstration or video guidance — correct technique is important for absorption.
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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.
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Mounjaro Dose Schedule And Titration
Mounjaro is started at a low dose (2.5 mg) and gradually increased over 20 weeks. You stay on each dose for a minimum of 4 weeks before escalating. The standard schedule is: 2.5 mg (weeks 1–4), 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), 10 mg (weeks 13–16), 12.5 mg (weeks 17–20), and up to 15 mg maximum (week 21 onwards). Your prescriber may adjust this based on how you respond and tolerate each dose. Never increase your dose earlier than 4 weeks, and never skip doses. If you experience side effects, contact your prescriber — they may recommend staying on your current dose longer before escalating.
Storage is critical for Mounjaro to work safely. Unopened pens must be kept in a refrigerator at 2–8°C and never frozen — if frozen, discard immediately. After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. Keep it away from direct heat and sunlight. Each KwikPen is used for exactly 4 weeks, then discarded safely — do not reuse pens. Write the start date on your pen so you remember when to dispose of it. Store your spare unopened pens in the fridge and bring them out to room temperature before use.
- 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.
What if I miss a weekly Mounjaro injection?
If you remember within 4 days of your missed dose, take it as soon as you can and resume your normal schedule the following week. If more than 4 days have passed, skip that dose and take your next dose on your normal scheduled day. Never take a double dose to catch up. Keep a calendar or phone reminder to help you stay on track with weekly injections.
Can I switch between injection sites each week?
Yes — rotating sites is recommended to prevent skin irritation, lumps, or lipodystrophy. You can use stomach, thigh, or upper arm, but change sites with each injection. Clean the skin with an alcohol wipe before injecting. If you notice redness, bruising, or swelling at a site, use a different site for at least a week.
How long does Mounjaro take to start working for weight loss?
Most people notice appetite suppression within 1–2 weeks of starting, though individual response varies. Significant weight loss typically becomes apparent after 4–8 weeks as you progress through doses. Clinical trials showed average weight loss of 15–21% over 72 weeks at maximum dose (15 mg), but results vary considerably between individuals.
What should I do if I accidentally inject into a blood vessel?
This is extremely unlikely with subcutaneous injection, but if you notice blood when injecting, remove the needle, apply pressure with a clean tissue, and contact your prescriber or pharmacist. You can use a fresh pen for that week's dose. This is a very rare occurrence — standard injection technique minimises this risk.
Does the time of day I inject Mounjaro matter?
No — you can inject at any time of day that suits your routine. Morning, afternoon, or evening all work equally well. The key is injecting once weekly on the same day of the week. Many people choose a fixed day and time for consistency, but Mounjaro works regardless of when you inject.
Ready to take control of your weight loss journey?
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