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 Bruising At The Injection Site — What You Need To Know
Bruising at the injection site is a common side effect when using Mounjaro (tirzepatide), a once-weekly weight loss injection prescribed in the UK. Small bruises, redness, or mild swelling at the injection area are typically normal and usually resolve within a few days. This guide explains what causes injection site bruising with Mounjaro, when it's considered normal, and what steps you can take to minimise it. If you're considering Mounjaro as part of your weight management journey, understanding these practical injection details can help you feel more confident about treatment.
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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.
What Causes Bruising At Mounjaro Injection Sites?
Bruising at injection sites occurs when the needle pierces small blood vessels under the skin, causing minor bleeding into surrounding tissue. This is not a sign that something has gone wrong—it's a common, generally harmless response to any injection. Factors that increase bruising risk include:
- Injecting into the same area repeatedly without rotating sites
- Injecting into areas with more blood vessels or thinner skin
- Taking blood-thinning medications (aspirin, warfarin)
- Low platelet count or bleeding disorders
- Pressing or massaging the injection site immediately after injection
- Using a needle at an angle rather than straight into the skin
- Bruising easily due to age, genetics, or nutritional deficiencies
When using your Mounjaro KwikPen, rotating injection sites each week significantly reduces bruising. The approved sites are your stomach (abdomen), upper thigh, and upper arm. Alternating between these locations allows previously injected areas to heal before you use them again.
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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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When Is Bruising Normal, And When Should You See Your GP?
Small bruises (1–2 cm across) that fade within 7 days are normal and require no treatment. Redness and mild swelling that resolve in 1–2 days are also typical. Contact your GP if:
- Bruising is very large, painful, or spreading rapidly
- The area develops signs of infection (warmth, pus, increasing redness beyond 48 hours)
- You experience severe pain at the injection site
- Bruising recurs at every injection despite rotating sites
- You have unexplained bruising elsewhere on your body
These situations may indicate an underlying issue that needs assessment. Your GP can review your injection technique, check for medication interactions, or assess whether Mounjaro is still appropriate for you. Serious injection site reactions are rare but should always be evaluated by a healthcare professional.
- 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.
How can I prevent bruising when using Mounjaro?
Rotate your injection sites every week between your stomach, upper thigh, and upper arm. Inject slowly and at a 90-degree angle. Avoid injecting into the same spot repeatedly. If you take blood-thinning medication, inform your GP before starting Mounjaro. Allow the injection site to dry completely if cleaned with antiseptic. Don't massage the area immediately after injection.
Is it normal to have a bruise after a Mounjaro injection?
Yes, small bruises are normal and common. They typically appear within hours and fade within 3–7 days. This happens because the needle pierces tiny blood vessels under the skin. As long as bruising is minor, not spreading, and the area isn't infected, you can continue your normal Mounjaro schedule.
What's the difference between a normal bruise and a sign of infection?
A normal bruise is flat, discoloured (purple, yellow, or green), and gradually fades. Infection signs include warmth, redness that spreads beyond 48 hours, pus, increasing pain, or swelling that worsens. If you notice these symptoms, contact your GP promptly, as infection needs medical evaluation.
Can I use my GLP-1 weight loss calculator if I'm experiencing injection site bruising?
Yes. Injection site bruising doesn't affect how Mounjaro works or change your projected weight loss results. Try our free GLP-1 weight loss calculator to see potential outcomes at different doses—bruising is a minor, temporary side effect that doesn't impact medication effectiveness.
Should I stop Mounjaro if I'm bruising at every injection?
No—stop and see your GP before your next dose. Frequent bruising despite site rotation may indicate you're taking blood thinners, have a bleeding disorder, or your injection technique needs adjustment. Your GP can assess whether continuing Mounjaro is safe and provide guidance on preventing further bruising.
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