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 Bruise — Normal Side Effect or Concern?
Bruising at the injection site is a common and usually harmless side effect of Mounjaro (tirzepatide), the once-weekly weight loss injection prescribed in the UK. Most bruises fade within a few days and don't require medical attention. However, understanding what's normal, what causes bruising, and how to prevent it can help you manage your treatment more confidently. This guide explains injection site reactions, when to contact your GP, and practical tips for reducing bruising with Mounjaro's KwikPen device.
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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.
Why Does Mounjaro Cause Bruising?
Bruising after a Mounjaro injection occurs because the needle penetrates small blood vessels under the skin, causing minor bleeding that appears as discolouration. This is a normal physiological response to any injection and is not a sign that something is wrong.
Several factors increase bruising risk with your weekly weight loss injection:
- Needle technique: Injecting too quickly, at the wrong angle, or inserting the needle too deeply can damage blood vessels. The Mounjaro KwikPen is designed for ease of use, but technique still matters.
- Thin skin or low body fat: Areas with less subcutaneous tissue (fat layer) bruise more easily because there's less cushioning between the needle and blood vessels.
- Blood thinning medications: If you take aspirin, warfarin, or other anticoagulants, you're more prone to bruising from any injection.
- Needle reuse: Always use a fresh needle for each Mounjaro dose — reused needles become bent or dulled, causing more tissue trauma.
- Injection site rotation: Injecting in the same spot repeatedly irritates tissue and increases bruising and redness.
Individual susceptibility varies — some people bruise easily from injections while others rarely show any marks.
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How It Works
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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Managing Bruising and Injection Site Reactions
Prevention strategies: Rotate your injection sites each week between your stomach, upper thigh, and upper arm (with assistance). Allow at least one week before re-injecting in the same spot. Check that your Mounjaro KwikPen needle is new and undamaged before each dose. Inject at a 90-degree angle into the fatty tissue layer, not muscle. Inject slowly and smoothly — rushing increases tissue damage.
If bruising occurs: Apply gentle ice for 10–15 minutes immediately after injection to reduce bleeding and swelling. Avoid massaging the area for at least 24 hours. Most bruises fade within 3–7 days. If bruising is severe, painful, or accompanied by swelling that doesn't improve, contact your GP or prescriber. Track your progress with our free progress tracker to monitor patterns and discuss with your healthcare provider if bruising consistently occurs at certain doses or injection sites.
- 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 bruising a common side effect of Mounjaro?
Yes, injection site bruising is common with Mounjaro. In clinical trials, injection site reactions (including bruising, redness, and swelling) occurred in up to 1 in 10 people. Most bruises are mild and fade within days without treatment. They're not dangerous, but proper injection technique and site rotation can minimise them significantly.
When should I contact my GP about Mounjaro bruising?
Contact your GP if bruising is severe, painful, or spreading beyond the immediate injection area; if swelling doesn't improve after a few days; if you develop signs of infection (warmth, pus, increasing redness); or if bruising repeatedly occurs despite rotating injection sites. These could indicate an underlying issue that needs assessment.
Can I prevent bruising with my Mounjaro KwikPen?
Yes. Always use a fresh needle each week — never reuse. Rotate injection sites weekly between stomach, thigh, and upper arm to avoid tissue irritation. Inject slowly at a 90-degree angle into fatty tissue, not muscle. Apply ice immediately after for 10–15 minutes. If you take blood-thinning medication, inform your prescriber, as they may recommend extra caution.
Should I stop Mounjaro if I bruise after injection?
No — injection site bruising alone is not a reason to stop treatment. Bruising is a common, harmless side effect that typically resolves quickly. Continue your weekly dose schedule unless your prescriber advises otherwise. If bruising is severe, painful, or accompanied by other symptoms, contact your healthcare provider for guidance.
Is there a difference between bruising and other injection site reactions?
Bruising is discolouration caused by bleeding under the skin. Other common injection site reactions include redness, swelling, itching, and tenderness. All are usually mild and self-limiting. Apply ice immediately after injection to reduce both bruising and swelling. If any reaction persists beyond a few days or worsens, contact your GP or prescriber.
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