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 — What You Need to Know
Bruising at the injection site is one of the most common side effects of Mounjaro injections, affecting many people using this tirzepatide medication. Bruising typically appears within a few hours of your weekly injection and usually fades within 7–10 days. Whilst bruising can be uncomfortable and sometimes unsightly, it is generally harmless and not a sign that anything is wrong with your medication. Understanding why bruising happens and how to minimise it can help you manage this side effect confidently.
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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 Does Mounjaro Cause Bruising?
Bruising after a Mounjaro KwikPen injection occurs because the needle penetrates blood vessels in the skin and underlying tissue, causing small amounts of bleeding beneath the surface. This is a normal physiological response to any injection. Several factors increase your bruising risk:
- Needle insertion technique — inserting the needle at the wrong angle or too quickly can damage blood vessels
- Injection site selection — some areas (inner thigh, stomach sides) have more superficial blood vessels than others
- Individual factors — blood clotting ability, skin thickness, age, and hydration status all affect bruising severity
- Medication interactions — if you take blood thinners (warfarin, aspirin) or NSAIDs, you may bruise more easily
- Skin fragility — older adults and those with certain connective tissue conditions bruise more readily
The good news is that injection-site bruising with Mounjaro is not a sign of an allergic reaction or medication problem — it is simply a mechanical response to the needle itself.
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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 Speed Recovery
Several practical steps can reduce bruising after your Mounjaro injection:
- Rotate injection sites weekly — alternate between stomach, thigh, and upper arm to prevent repeated damage to the same area
- Let the pen reach room temperature — cold medication can make blood vessels more fragile
- Inject slowly — press the button smoothly rather than jabbing quickly
- Avoid rubbing the site — do not massage the injection area for at least 10 minutes afterwards
- Apply ice for 5 minutes before — numbs the area and constricts blood vessels
- Apply warmth afterwards — gentle heat can promote blood flow and reduce pooling
- Stay hydrated — well-hydrated skin is more resilient
If bruising persists beyond two weeks or is severe, contact your GP or the prescriber who provided your Mounjaro UK prescription.
- 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 side effect of Mounjaro?
Yes, bruising at the injection site is listed as a common side effect, affecting up to 1 in 10 people. It occurs because the needle causes small amounts of bleeding under the skin. Bruising is not dangerous and usually fades within 7–10 days.
Can I prevent bruising from Mounjaro injections?
You cannot eliminate bruising entirely, but you can reduce it by rotating injection sites, allowing the pen to reach room temperature, injecting slowly, avoiding site massage, and staying well hydrated. Using ice before injection may also help constrict blood vessels.
What should I do if I have severe bruising after Mounjaro?
Mild to moderate bruising is normal and expected. However, if bruising is severe, spreads rapidly, is accompanied by swelling or warmth, or persists beyond two weeks, contact your GP or prescriber. These could indicate a vascular issue requiring assessment.
Does bruising mean my Mounjaro dose is wrong?
No. Bruising from Mounjaro injections is a local skin reaction to the needle itself, not related to the medication dose or effectiveness. Your dose is determined by your prescriber based on your clinical assessment, not injection-site bruising.
How does Mounjaro bruising compare to Wegovy?
Both Mounjaro and Wegovy can cause injection-site bruising because both are weekly injections. Bruising rates are similar — it is a common side effect of both medications. Your individual bruising risk depends on skin type, technique, and blood vessel fragility rather than which medication you use.
Ready to take control of your weight loss journey?
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