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 and Bruising — What You Need to Know
Bruising at the injection site is a common side effect when taking Mounjaro (tirzepatide), the once-weekly weight management injection. Most bruising is mild and resolves within a few days, but it's helpful to understand what's normal and when to seek guidance from your GP. This guide explains why bruising happens with Mounjaro, how to minimise it, and what to watch for. Our free GLP-1 weight loss calculator can help you explore potential outcomes with this medication.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 Bruising Happens with Mounjaro Injections
Bruising at the injection site occurs when small blood vessels (capillaries) under the skin are damaged during the needle insertion. This is a normal physiological response and is listed as a common side effect in Mounjaro's UK prescribing information. The KwikPen needle is very fine, so bruising is typically minor, but individual responses vary depending on skin sensitivity, blood vessel fragility, and how the injection is administered.
- Bruising may appear as a small red mark immediately after injection, then darken to blue or purple over 24–48 hours
- It usually fades within 7–14 days without treatment
- Larger bruises are more common if you inject into an area with recent bruising or hit a small blood vessel
- Rotating injection sites helps reduce repeated trauma to the same area
If you experience severe or unusually large bruising, or if bruising persists beyond two weeks, inform your GP or the healthcare professional who prescribed your Mounjaro.
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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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How to Minimise Bruising and When to Contact Your GP
Practical steps to reduce bruising: Ensure your skin is clean and dry before injecting. Let the KwikPen reach room temperature if it's been refrigerated. Inject slowly and at a 90-degree angle. Avoid injecting into areas with existing bruises or scars. After injection, gently massage the area for a few seconds — do not rub vigorously. If bruising is accompanied by severe pain, swelling, redness spreading beyond the immediate injection site, warmth, or signs of infection, contact your GP promptly as these may indicate a complication. Most bruising is harmless, but your GP can assess whether any treatment or injection technique adjustment is needed.
- 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 normal side effect of Mounjaro?
Yes. Bruising at the injection site is listed as a common side effect in UK prescribing information for Mounjaro. Most bruising is minor and resolves within 7–14 days. It's a normal response to needle insertion and does not indicate that the medication is unsafe or ineffective.
How long does Mounjaro bruising typically last?
Most injection site bruising fades within 7–14 days. Small bruises may disappear within a few days, while larger bruises can take up to two weeks. If bruising persists beyond two weeks, is unusually large, or is accompanied by pain or swelling, contact your GP for assessment.
Can I prevent bruising when using the Mounjaro KwikPen?
You can reduce bruising risk by rotating injection sites weekly, ensuring the pen is at room temperature, injecting at a 90-degree angle, avoiding recent bruise sites, and using gentle technique. However, some bruising is common and cannot always be prevented, especially in people with more fragile blood vessels.
Should I contact my GP about Mounjaro bruising?
Mild bruising does not require GP contact. However, contact your GP if bruising is severe, unusually large, persists beyond two weeks, spreads significantly, or is accompanied by pain, swelling, redness, warmth, or signs of infection. These may indicate a complication or injection technique issue.
Does Mounjaro bruising mean the injection didn't work?
No. Bruising is a minor injection site reaction unrelated to how effectively Mounjaro works. The medication's effectiveness in supporting weight loss is determined by how your body responds to tirzepatide, not by whether you develop injection site bruising.
Ready to take control of your weight loss journey?
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