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 Bruising — What You Need to Know
Injection site bruising is one of the most common side effects when using Mounjaro (tirzepatide), the weekly weight loss injection available through UK specialist services and private prescribing. Bruising typically develops after injecting into the stomach, thigh, or upper arm and usually fades within 7–14 days. Whilst generally harmless, understanding what causes bruising, when it's normal, and how to prevent it can help you manage this side effect confidently and continue your weight loss journey with minimal discomfort.
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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 Mounjaro Injections Cause Bruising
Bruising occurs because the needle punctures small blood vessels under the skin, causing minor bleeding into surrounding tissue. This is a normal physiological response and happens with many injectable medications. Several factors increase your bruising risk:
- Needle technique: Injecting too quickly, at the wrong angle, or with excessive force can damage blood vessels
- Injection site: Areas with more blood vessels (inner thigh, stomach) bruise more easily than others
- Medications: If you take blood thinners (aspirin, warfarin) or NSAIDs (ibuprofen, naproxen), your risk increases significantly
- Individual factors: Some people naturally bruise more easily due to genetics, age, or vitamin K deficiency
- Repeated injections: Using the same site weekly without rotating can accumulate bruising
As a prescription-only weekly weight loss injection, Mounjaro requires proper injection technique—your GP or prescriber should provide clear guidance when you start treatment.
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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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How to Prevent and Manage Bruising
Prevention strategies: Rotate your injection site every week to allow bruised areas to heal. Inject into your stomach, then outer thigh, then upper arm in rotation. Let the skin warm to room temperature before injecting if the pen has been refrigerated. Inject slowly and smoothly at a 90-degree angle. Apply gentle pressure with a clean cloth for 10 seconds after injecting, but do not massage the area as this can increase bruising. If you take blood thinners, inform your prescriber before starting Mounjaro, as they may recommend additional precautions.
Managing existing bruises: Apply a cold compress for the first 24 hours, then switch to a warm compress. Mild bruising typically resolves within two weeks without treatment. If bruising is severe, spreading, or accompanied by swelling, redness, or pain, contact your GP for advice.
- 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 Mounjaro injection site bruising dangerous?
No. Minor bruising is a normal, harmless side effect of injectable medications. Bruises typically fade within 7–14 days without treatment. However, if you notice severe bruising, spreading inflammation, signs of infection (warmth, pus, fever), or bruising that doesn't improve after two weeks, contact your GP for assessment.
Can I take paracetamol or ibuprofen for injection site bruising?
Paracetamol is safe for bruising discomfort. Avoid NSAIDs like ibuprofen unless advised otherwise by your GP, as they can increase bleeding and worsen bruising. If you already take NSAIDs regularly, inform your prescriber before starting Mounjaro so they can monitor you.
How often should I rotate my Mounjaro injection sites?
Rotate your injection site every week. Use your stomach one week, outer thigh the next, upper arm the following week, then rotate back to stomach. This gives each area 3 weeks to heal between injections and significantly reduces bruising accumulation over time.
Does Mounjaro bruising affect weight loss results?
No. Injection site bruising does not affect how Mounjaro works or your weight loss progress. The medication's effectiveness depends on proper dosing, dose titration, and lifestyle factors—not on bruising. Use our free GLP-1 weight loss calculator to track your projected results.
Should I stop using Mounjaro if I'm bruising badly?
Do not stop without consulting your prescriber. If bruising is severe, discuss it with your GP or the prescriber who issued your UK prescription. They may suggest injection technique adjustments, additional monitoring if you take blood thinners, or alternative sites. Severe bruising is rare and usually preventable with proper technique.
Ready to take control of your weight loss journey?
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