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 Use Mounjaro in the UK: Dosing and Injection Guide
Mounjaro (tirzepatide) is a prescription-only weight loss injection used in the UK, delivered via a pre-filled KwikPen device once weekly. If you've been prescribed Mounjaro, understanding the correct dosing schedule, injection technique, and storage is essential for safe and effective use. This guide covers everything UK users need to know about using Mounjaro properly, plus how it compares to Wegovy for weight management. Always follow your prescriber's instructions and contact your GP if you have questions about your treatment.
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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.
Mounjaro Dosing Schedule and How to Inject
Mounjaro is administered as a once-weekly injection using a pre-filled KwikPen. The standard dose escalation follows a stepwise schedule: start with 2.5 mg for weeks 1–4, then increase to 5 mg for weeks 5–8, then 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg (the maximum dose). Your prescriber will guide you through the titration schedule based on your individual response and tolerance.
- Each KwikPen contains four doses (four weeks' supply)
- Inject on the same day each week for consistency
- You can inject in your stomach, thigh, or upper arm
- Rotate injection sites each week to prevent skin irritation
- Store unopened pens in the fridge at 2–8°C
- After first use, pens can be kept at room temperature (below 30°C) for up to 30 days
If you miss a dose within 4 days, inject as soon as you remember and resume your normal schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day.
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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 vs Wegovy: Key Differences in Use
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only, activating a single pathway. In clinical trials, Mounjaro showed an average weight loss of approximately 15–21% of body weight, whilst Wegovy showed around 14.9% at its maximum dose—though individual results vary significantly.
Dosing differs: Mounjaro starts at 2.5 mg and escalates weekly to a maximum of 15 mg, whilst Wegovy starts at 0.25 mg and escalates to 2.4 mg over a longer period. Both are once-weekly injections. Your prescriber will assess which medication is more suitable for you based on your health profile, preferences, and clinical response.
- 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 is the Mounjaro KwikPen and how do I use it?
The Mounjaro KwikPen is a pre-filled injection device that delivers your weekly dose of tirzepatide. Simply select your dose dial on the pen, pinch skin on your stomach, thigh, or arm, insert the needle at 90 degrees, press the button, and hold for 3–6 seconds. Your prescriber will demonstrate the correct technique before you start.
How long does a Mounjaro KwikPen last?
Each KwikPen contains four weekly doses, so one pen lasts four weeks. You'll need to order your next pen before it runs out. If you're prescribed through the NHS, your specialist service will arrange repeat supplies. If private, your prescriber or pharmacy will manage your refills.
Can I switch from Mounjaro to Wegovy?
Switching between Mounjaro and Wegovy is possible but must be managed by your prescriber. There is no official conversion chart—your prescriber will assess your individual response and recommend an appropriate starting dose for Wegovy based on your current Mounjaro dose. Never stop and start medications without medical guidance.
What if I miss a Mounjaro injection?
If you remember within 4 days of your missed dose, inject as soon as possible and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and resume your next scheduled injection. Maintain at least 72 hours between injections. Contact your prescriber if you're unsure.
Does Mounjaro work better than Wegovy for weight loss?
Clinical trial data suggests Mounjaro may produce slightly larger average weight losses than Wegovy, but individual results vary significantly. Which medication is "better" depends on your personal response, side effects, and health profile. Your prescriber will recommend the most suitable option for your individual circumstances based on clinical assessment.
Ready to take control of your weight loss journey?
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