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 After Your Mounjaro Injection
Bruising at the injection site is one of the most common side effects people experience with Mounjaro, the weekly weight loss injection used in the UK for obesity and overweight management. Most bruises are mild, fade within a few days, and are nothing to worry about. However, understanding why bruising happens, how to reduce it, and when to contact your GP can help you manage this side effect confidently as part of your weight loss journey.
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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 Bruising Happens With Mounjaro and Wegovy Injections
Bruising occurs because the needle punctures tiny blood vessels under the skin during injection. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are weekly weight loss injections delivered via pre-filled KwikPen devices, and both can cause injection site bruising. The difference is subtle: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only, but both mechanisms work via injection.
- Minor blood vessel damage is normal during any injection
- Some people bruise more easily due to genetics, age, or blood thinning medications
- Bruising typically appears within hours and fades within 7–14 days
- Rotating injection sites (stomach, upper thigh, upper arm) can help reduce repeated bruising in one area
- Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) trials did not show bruising as a major safety concern
If you're considering a UK prescription weight loss injection, our free GLP-1 weight loss calculator can help you understand what to expect at different doses and stages of 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.
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How to Prevent and Manage Bruising at Injection Sites
Prevention strategies: Allow the KwikPen to reach room temperature before injecting, as cold injections can irritate tissue. Pinch the skin gently but do not rub the injection site immediately afterwards. Always rotate where you inject each week—avoid the same spot repeatedly. If you're on aspirin or other blood thinners, inform your prescriber before starting Mounjaro or any weekly weight loss injection, as this may increase bruising risk.
Managing bruises: Apply ice for the first 10 minutes after injection if bruising appears quickly. After 48 hours, gentle warmth (a warm flannel) may help. Avoid squeezing or massaging the area. Most bruises resolve naturally within 1–2 weeks without treatment. Use our free BMI calculator and weight converter to monitor your progress independent of injection site appearance.
When to contact your GP: Bruising alone does not usually require medical attention. However, seek advice if you experience severe pain at the injection site, large areas of bruising (larger than 5cm) that worsen after 48 hours, signs of infection (warmth, redness, pus, fever), or if bruising is accompanied by swelling that restricts movement. These could indicate complications beyond simple injection site bruising and your GP can assess whether to continue Mounjaro therapy.
- 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 after Mounjaro injection normal?
Yes, mild bruising is very common with Mounjaro and Wegovy injections. Most bruises appear within hours, are painless, and fade within 1–2 weeks. Bruising occurred in clinical trials but was not classed as a serious safety concern. If bruising is severe or worsening, contact your GP.
Does Wegovy cause more bruising than Mounjaro?
Both are weekly injections delivered via similar pen devices, so bruising rates are comparable. Individual variation is normal—some people bruise more easily due to genetics, medications, or skin sensitivity. Your prescriber can advise on injection technique to minimise bruising.
How can I reduce bruising from my weight loss injection?
Allow the pen to warm to room temperature, rotate injection sites weekly (stomach, thigh, upper arm), avoid rubbing the site immediately after injection, and do not inject into bruised areas. If you take blood thinners, inform your prescriber. Gentle ice in the first 10 minutes may help.
Should I stop Mounjaro if I'm bruising a lot?
Not necessarily. Bruising alone is not a reason to stop treatment. However, if bruising is severe, painful, or accompanied by infection signs (warmth, redness, pus), contact your GP. They can assess whether the bruising indicates an injection technique issue or an underlying concern.
How long does bruising last after a Mounjaro injection?
Most injection site bruises fade within 7–14 days. Larger bruises may take 2–3 weeks. After 48 hours, gentle warmth can support healing. If bruising persists beyond 3 weeks or worsens, seek GP advice to rule out other causes.
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