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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Bruising From Mounjaro — What You Should Know
Bruising at the injection site is one of the most common side effects experienced by people taking Mounjaro (tirzepatide) in the UK. It's typically mild and temporary, but understanding why it happens and how to manage it can help you feel more confident during your weight management treatment. This guide explains the causes, what's normal, and when to contact your GP.
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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?
Mounjaro is delivered via a once-weekly injection using a KwikPen device, and bruising occurs when the needle punctures small blood vessels under the skin. This is a normal physiological response and doesn't mean the injection has been given incorrectly. Several factors increase bruising risk: injecting into the same site repeatedly without rotating, injecting into areas with thinner skin, taking anticoagulant medications (such as aspirin or warfarin), or having a bleeding disorder. The needle used in the KwikPen is very fine, but even tiny blood vessels can leak slightly when punctured. Bruising is listed as a common side effect in the UK product information, affecting up to 1 in 10 people. Most bruises appear within hours of injection and typically fade within 1–2 weeks without treatment.
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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 Minimise Bruising — Practical Tips
Rotate your injection sites every week — use your stomach (abdomen), upper thigh, or upper arm in turn, and vary the exact location within each area. This reduces repeated trauma to the same site. Apply ice to the area for 5 minutes before injecting to constrict blood vessels and reduce bleeding. After injection, apply gentle pressure for 10 seconds but avoid rubbing the area. Do not massage the injection site, as this can worsen bruising. If you take aspirin, ibuprofen, or other blood-thinning medications, discuss this with your GP before starting Mounjaro — dose timing may be adjusted. Ensure your technique is correct: inject at a 90-degree angle and hold the skin steady. If bruising is severe or accompanied by swelling or warmth, contact your GP or pharmacist to rule out infection.
- 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 from Mounjaro dangerous?
No. Bruising is a normal, harmless injection site reaction. Small blood vessels under the skin rupture when the needle is inserted, causing temporary discolouration. Bruises typically fade within 1–2 weeks. Seek help only if bruising is severe, spreading, or accompanied by warmth, redness, or pus — these could indicate infection.
Can I prevent bruising completely?
You cannot prevent bruising entirely, but you can significantly reduce it. Rotate injection sites weekly, apply ice before injecting, avoid massaging the area after injection, and use correct injection technique. If you take blood-thinning medications, discuss timing with your GP. Some people naturally bruise more easily due to genetics or medications.
What's the difference between normal bruising and a problem?
Normal Mounjaro bruising is discolouration only — it may be blue, purple, yellow, or green, but is painless and fades in 1–2 weeks. A problem exists if you experience: severe swelling that limits movement, warmth or heat at the site, pus or discharge, increasing redness spreading beyond the bruise, or pain that worsens. Contact your GP if any of these occur.
Should I tell my GP about bruising from Mounjaro?
Mild bruising does not require reporting — it's an expected, common side effect. However, inform your GP if bruising is severe, recurrent at every injection, accompanied by other symptoms, or if you're concerned. Your GP can advise on injection technique or recommend strategies. If you're taking Mounjaro through the NHS via a tier 3 specialist service, discuss persistent bruising at your next review.
Does bruising mean Mounjaro is working?
No. Bruising is a mechanical side effect of injection, not a sign of effectiveness. Whether Mounjaro is working depends on your weight loss progress over weeks and months. Track your progress using our free progress tracker to monitor actual weight loss outcomes. Calculate your BMI using our BMI calculator to establish your baseline and measure improvements over time.
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