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 — What to Expect
Injection site reactions are among the most common side effects of Mounjaro (tirzepatide), a prescription-only weight loss injection licensed in the UK. These reactions are typically mild and often improve as your body adjusts to the medication. This guide explains what injection site reactions are, why they happen, how to recognise them, and what you should do if they become uncomfortable or persistent. Understanding what to expect can help you feel more confident about using this once-weekly weight loss injection.
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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.
What Are Mounjaro Injection Site Reactions?
Injection site reactions are localised responses where you receive your weekly Mounjaro injection. They are very common — occurring in up to 1 in 10 people taking Mounjaro — and are generally mild and short-lived. Common injection site reactions include redness, bruising, swelling, itching, pain, or a small bump or lump at the injection site.
- Redness: The skin around the injection may appear pink or red, usually fading within hours to days
- Bruising: Minor purple or blue discolouration, similar to a routine blood test or vaccine
- Swelling: Mild puffiness around the injection area that typically resolves quickly
- Itching or tenderness: A mild itch or sore feeling when touching the area
Most injection site reactions resolve within 24 to 48 hours without treatment. Unlike systemic side effects (such as nausea or diarrhoea), injection site reactions are localised and do not affect the rest of your body. They are a sign that your body is responding to the injection, not that anything is wrong.
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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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When to Seek Medical Advice
Whilst most injection site reactions are harmless, you should contact your GP or the prescriber who issued your Mounjaro if:
- The reaction persists for more than a few days or worsens
- The affected area becomes increasingly hot, red, or swollen (may indicate infection)
- You develop pus, discharge, or an open wound at the injection site
- You experience severe pain that does not improve with over-the-counter painkillers
- You have signs of a serious allergic reaction (swelling of lips, tongue, or face)
Serious injection site infections are rare when proper injection technique and hygiene are followed. If you are unsure whether your reaction requires medical attention, contact your GP or call NHS 111 for guidance. Your prescriber can also assess whether continuing Mounjaro is appropriate for you.
- 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.
How long do Mounjaro injection site reactions usually last?
Most injection site reactions resolve within 24 to 48 hours. Bruising or mild redness may take slightly longer but typically fade within a week. If your reaction persists beyond a few days or worsens, contact your GP.
Can I reduce injection site reactions?
Yes. Rotate your injection site each week (stomach, thigh, or upper arm), ensure the injection pen is at room temperature before use, and clean the skin with an alcohol wipe beforehand. Avoid injecting into areas that are already bruised or tender.
Is an injection site reaction a sign that Mounjaro isn't working?
No. Injection site reactions are a localised response and do not indicate whether Mounjaro is working for weight management. The medication's effectiveness is measured by weight loss over weeks and months, not by injection site effects. Use our GLP-1 weight loss calculator to track your potential progress.
Should I stop taking Mounjaro if I get a reaction?
Most reactions do not require stopping the medication. However, if the reaction is severe, infected, or accompanied by signs of serious allergy, contact your prescriber immediately. Never stop a prescription medicine without medical guidance.
Are injection site reactions more common with higher Mounjaro doses?
Injection site reactions are not typically dose-dependent — they occur at all dose levels from 2.5mg through to 15mg. If you experience persistent reactions, your prescriber may suggest rotating injection sites more carefully or adjusting your injection technique.
Ready to take control of your weight loss journey?
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