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 Inject Mounjaro — Safe Technique Guide
Mounjaro (tirzepatide) is a once-weekly GLP-1 receptor agonist injection used for weight management in UK adults. If you've been prescribed Mounjaro, understanding the correct injection technique is essential for safety and effectiveness. This guide covers step-by-step instructions, storage requirements, and how the injection process compares to Wegovy. Always follow your prescriber's instructions and the patient information leaflet included with your medication.
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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 Injection Technique vs Wegovy — Key Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injections delivered using pre-filled pens, but they have important differences in mechanism and delivery. Mounjaro is a dual GIP receptor and GLP-1 receptor agonist — meaning it works on two hormone pathways — whereas Wegovy is a GLP-1 receptor agonist only. Clinical trial data shows different profiles: Mounjaro (SURMOUNT trial) demonstrated average weight loss of 15–21% body weight, while Wegovy (STEP-1 trial) showed approximately 15% average body weight loss. Both require weekly injections and similar injection technique, but Mounjaro doses escalate from 2.5 mg to a maximum of 15 mg, while Wegovy escalates from 0.25 mg to 2.4 mg. Individual suitability depends on your health profile, and your prescriber will assess which is appropriate following a clinical assessment.
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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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Step-by-Step Mounjaro Injection Instructions
Before injecting: Remove your KwikPen from the refrigerator 15 minutes beforehand — allowing it to reach room temperature reduces discomfort. Check the expiry date and that the liquid appears clear and colourless. Wash your hands and choose an injection site on your stomach (abdomen), upper thigh, or upper arm (if an adult is helping). Rotate sites weekly to prevent skin problems.
- Pinch the skin firmly at your chosen site
- Insert the needle at a 90-degree angle — push down until you hear or feel a click
- Keep the pen in place for 6 seconds to ensure the full dose delivers
- Remove the needle and release the skin
- Do not rub the area after injection
After use, store the pen in the refrigerator if unopened, or at room temperature (below 30°C) for up to 30 days after first 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.
Can I inject Mounjaro myself, or do I need help?
Most people can self-inject Mounjaro using the pre-filled KwikPen. However, if you're uncomfortable with self-injection or choose the upper arm site, a healthcare professional or trusted adult can assist. Your prescriber or pharmacist will demonstrate the technique during your initial consultation.
What's the difference between Mounjaro and Wegovy injection pens?
Both use pre-filled pens and once-weekly injection. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist starting at 2.5 mg, while Wegovy (semaglutide) is a GLP-1 agonist only, starting at 0.25 mg. Dose ranges and escalation schedules differ. Both require a prescription and clinical assessment to determine suitability.
How long does a Mounjaro injection take?
The actual injection takes 6 seconds. Total preparation time — including removing the pen from refrigeration, checking the dose, and preparing the injection site — typically takes 5–10 minutes. Plan a few minutes before your scheduled injection time.
What should I do if I miss a Mounjaro injection?
If you remember within 4 days of your scheduled injection, take it as soon as possible and continue your regular schedule the following week. If more than 4 days have passed, skip that dose and inject on your normally scheduled day. Never take a double dose. Contact your prescriber if you're unsure.
Can I switch from Mounjaro to Wegovy, or vice versa?
Switching between Mounjaro and Wegovy requires prescriber guidance — there's no official established conversion chart, though approximate equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy). Your prescriber will manage any switch with appropriate dose adjustments. Always discuss this with your GP or the prescriber issuing your prescription.
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