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 Administer Mounjaro: Step-by-Step UK Guidance
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. If you've been prescribed Mounjaro on the NHS or through a private UK prescriber, understanding the correct injection technique is essential for safety and effectiveness. This guide walks you through the administration process, from preparing your pen to choosing injection sites, plus how it compares to administering Wegovy for those weighing up weight loss options.
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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 vs Wegovy: Administration and Mechanism Differences
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both once-weekly injections for weight management in UK adults, but they work differently and have distinct administration considerations.
Key differences:
- Mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only — this dual action in Mounjaro may support different appetite regulation pathways
- Dosing range: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg. Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg
- Injection device: Both use pre-filled pens, but the needle gauge and pen design differ slightly
- Clinical trials: SURMOUNT trials showed Mounjaro average weight loss of 15–21% body weight; STEP trials showed Wegovy average loss of approximately 15% — though individual results vary significantly
Both are prescription-only medicines requiring a clinical assessment before being prescribed. Your prescriber will determine which is suitable based on your individual health profile.
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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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How to Inject Mounjaro: Safe Administration Steps
Mounjaro is injected once weekly, every 7 days. Before injecting: Remove the KwikPen from the fridge 15 minutes before use to allow it to warm to room temperature. Check that the liquid is clear and colourless — do not use if it looks cloudy or discoloured.
Injection sites: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to prevent skin irritation. Pinch the skin, insert the needle at 90 degrees, press the button, and hold for 3 seconds. After injecting, dispose of the pen safely in a designated sharps bin.
If you miss a dose within 4 days of your scheduled injection date, take it as soon as you remember. If more than 4 days have passed, skip that dose and continue your normal weekly schedule. Never take a double dose.
- 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.
Is administering Mounjaro painful?
Most people report minimal discomfort. The needle is very fine (27 gauge). Pain varies by individual and injection site — thigh injections often feel less uncomfortable than stomach injections for some people. If you experience significant pain, speak to your prescriber or pharmacist about technique.
Can I switch from Mounjaro to Wegovy, or vice versa?
Switching between medications must be managed by your prescriber. Mounjaro and Wegovy are not directly equivalent — approximate dose conversions exist (5 mg Mounjaro ≈ 0.5 mg Wegovy), but any switch requires professional guidance to ensure safe transition and appropriate dosing.
How long can Mounjaro stay out of the fridge after opening?
After your first injection, the KwikPen can be stored at room temperature (below 30°C) for up to 30 days. Dispose of the pen after 30 days, even if some medication remains. Always keep it away from direct heat and sunlight.
What should I do if I accidentally inject Mounjaro twice in one week?
If you accidentally take two doses within 7 days, contact your GP or pharmacist immediately for advice. Do not inject again — wait until your next scheduled injection date (7 days from the last dose). Your prescriber may advise on monitoring for side effects.
Can I use the same injection site every week?
No — always rotate your injection site each week to prevent lipohypertrophy (thickened skin). Acceptable sites include stomach, upper thigh, and upper arm. Varying your site also helps reduce injection-related discomfort and irritation over time.
Ready to take control of your weight loss journey?
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