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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Mounjaro Injection Site Reactions — Managing Treatment
Injection site reactions are common when using Mounjaro (tirzepatide), the once-weekly weight loss injection licensed in the UK. Most reactions are mild and temporary, including redness, bruising, swelling, or pain at the injection site. Understanding how to manage these reactions and when to seek help from your GP can help you continue your treatment confidently. This guide covers NHS-aligned guidance on recognising, treating, and preventing injection site reactions whilst using this prescription-only 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.
Understanding Mounjaro Injection Site Reactions
Injection site reactions occur in up to 1 in 10 people using Mounjaro and are typically mild and temporary. Common reactions include redness, slight bruising, small lumps under the skin, itching, or tenderness that usually resolve within a few days. These reactions happen because the injection needle creates a small puncture in the skin and the medication itself may cause localised inflammation.
- Immediate reactions: Redness or mild pain that appears right after injection
- Delayed reactions: Bruising or swelling that develops over 24–48 hours
- Persistent reactions: Itching or small lumps lasting more than a week
Most injection site reactions are not serious and do not require you to stop your Mounjaro treatment. However, if you develop signs of infection (warmth, pus, or fever) or reactions that worsen, contact your GP or NHS 111 for advice.
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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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Managing and Preventing Injection Site Reactions
To minimise injection site reactions, rotate your injection sites weekly between your stomach, upper thigh, and upper arm—never inject into the same spot twice in a row. Allow the skin to air dry completely after cleaning with an alcohol wipe, and ensure your KwikPen is at room temperature before injecting. For mild reactions, apply a cold compress for 10 minutes or use paracetamol as directed on the packet.
If you experience persistent itching or discomfort, your GP can recommend a suitable moisturiser or antihistamine. Always inform your prescriber of any reactions so they can monitor your treatment. Use our GLP-1 weight loss calculator to track your progress whilst managing any side effects—understanding your expected weight loss journey can help you stay motivated through minor injection reactions.
- 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.
Are Mounjaro injection site reactions serious?
Most injection site reactions are mild and not serious—redness, bruising, and mild pain typically resolve within days. However, if you develop signs of infection (warmth, increasing pain, pus, or fever), contact your GP promptly. Serious allergic reactions affecting your face, lips, or throat require immediate medical attention.
How can I prevent injection site reactions with Mounjaro?
Rotate injection sites weekly between stomach, upper thigh, and upper arm to prevent repeated trauma to the same skin area. Ensure the KwikPen is at room temperature, let cleaned skin dry completely, and use proper injection technique. Most people experience fewer reactions as their skin adapts over several weeks.
What should I do if I have a bruise after my Mounjaro injection?
Small bruises are normal and usually fade within 7–14 days without treatment. Apply a cold compress immediately after injection (10 minutes) to reduce bruising. Avoid rubbing the area and do not massage the injection site. If bruising is severe or accompanied by swelling, contact your GP for advice.
Can I continue Mounjaro if I have injection site reactions?
Yes, mild injection site reactions do not require you to stop Mounjaro. Contact your GP if reactions persist beyond one week or if you're concerned about your symptoms. Your prescriber can advise whether to continue, adjust your technique, or use different injection sites to minimise reactions.
When should I contact my GP about an injection site reaction?
Contact your GP if you develop signs of infection (increasing warmth, pus, fever), severe swelling that spreads beyond the injection site, persistent allergic symptoms, or reactions that worsen over time. For immediate concerns about serious allergic reactions, call NHS 111 or attend an A&E department.
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