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
Free tracker included with every download this week
Why Does Mounjaro Cause Injection Site Bruising?
Injection site bruising is a common side effect when using Mounjaro (tirzepatide), the prescription-only weekly weight loss injection licensed in the UK. Bruising occurs because the needle penetrates small blood vessels under the skin during injection. While usually harmless and temporary, understanding how to reduce bruising and what's normal can help you feel more confident with your treatment. Most bruising fades within days to weeks without intervention.
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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.
How to Minimise Bruising with Mounjaro Injections
Several practical steps can help reduce bruising when using your Mounjaro KwikPen:
- Rotate injection sites weekly: Alternate between your stomach (abdomen), upper thigh, and upper arm. This gives tissues time to heal and prevents repeated trauma to the same area.
- Allow the pen to reach room temperature: Cold insulin or medication can cause more vascular constriction. Remove from the fridge 15–20 minutes before injection.
- Inject slowly and steadily: Rushing the injection increases tissue trauma. Hold the pen firmly in place for 3 seconds after pressing the button.
- Avoid injecting into bruised areas: Wait until bruising has completely faded before reusing that site.
- Apply gentle pressure after injection: Use a clean cotton pad to apply light pressure for a few seconds—do not rub.
- Stay hydrated: Well-hydrated skin and tissues are more resilient and bruise less easily.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When to Contact Your GP or Prescriber
Most bruising from Mounjaro is mild, painless, and resolves without treatment. However, contact your GP or prescriber if you experience severe bruising, swelling that worsens, redness spreading beyond the injection site, warmth or heat at the site (suggesting infection), or persistent pain or numbness. These symptoms may indicate an infection, allergic reaction, or nerve involvement—all of which need clinical assessment. If you're concerned about any injection site reaction, it's always better to seek advice early. Your prescriber can assess whether you need to adjust your injection technique or switch to a different site.
- 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 injections normal?
Yes, injection site bruising is very common when using Mounjaro. It occurs because the needle penetrates small blood vessels under the skin. Most bruising is mild, painless, and fades within days to weeks. It's not a sign of improper injection unless accompanied by severe swelling, redness, or infection signs.
How long does Mounjaro injection site bruising last?
Most bruising fades within 3–7 days, though larger bruises may take 1–2 weeks to disappear completely. The timeline depends on bruise severity and individual healing. Applying ice in the first 10 minutes can help reduce swelling, while heat after 48 hours may speed healing.
Can I prevent bruising from my weekly Mounjaro injection?
Yes. Rotating injection sites weekly, allowing the pen to warm to room temperature, injecting slowly, avoiding bruised areas, applying gentle pressure after injection, and staying well-hydrated all help minimise bruising. These practical steps significantly reduce the risk of severe bruising over time.
Should I be worried about bruising with Mounjaro?
Mild bruising is not concerning and doesn't mean your injection is ineffective. However, contact your prescriber if you notice severe swelling, spreading redness, warmth, signs of infection, or persistent pain. These symptoms may require clinical assessment.
Is Mounjaro bruising different from Wegovy bruising?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections and can cause similar injection site bruising. Both are prescription-only weight loss medications licensed in the UK. Bruising risk is comparable; minimisation strategies are the same for both medications.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.