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 Mounjaro injection sites is a common and usually harmless side effect. Many people taking this prescription tirzepatide injection experience mild redness, swelling, or bruising where they inject. Whilst it can look concerning, injection site bruising typically resolves within a few days and doesn't mean the medication isn't working. This guide explains why bruising happens, when to seek advice from your GP, and practical steps to minimise it during your Mounjaro 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.
Why Does Mounjaro Cause Bruising at Injection Sites?
Bruising at Mounjaro injection sites occurs because the needle penetrates small blood vessels just under the skin. Each time you inject, there's a small amount of bleeding into the surrounding tissue, which the body reabsorbs over several days. This is completely normal and not a sign of an injection technique problem.
Several factors increase bruising risk:
- Injection technique — inserting the needle too quickly or at the wrong angle can damage more blood vessels
- Thin skin — areas with less fatty tissue bruise more easily
- Blood thinners — medications like aspirin or warfarin increase bleeding under the skin
- Repeated injection sites — reusing the same spot week after week prevents healing
- Vitamin K deficiency — rare, but affects clotting and bruising
The Mounjaro KwikPen is designed for subcutaneous injection (into fatty tissue), which minimises nerve and blood vessel damage compared to other injection depths. Most bruising is mild and fades within 3–7 days without 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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Managing Mounjaro Injection Site Bruising
Practical steps to reduce bruising include rotating injection sites each week across your stomach, thighs, and upper arms. Allow at least one week between injections in the same spot. Before injecting, let the Mounjaro pen reach room temperature naturally (do not warm it artificially). Inject slowly and smoothly, holding the needle in place for 3 seconds after pressing the plunger to allow medicine to disperse evenly.
After injecting, apply light pressure with a clean cotton ball for 10 seconds—avoid rubbing. You can apply a cool compress for 5–10 minutes if bruising develops. If you take blood thinners, discuss your Mounjaro dose and injection technique with your GP or pharmacist. Most bruising fades naturally; silicone-based scar creams may help cosmetically but aren't medically necessary.
- 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 injection sites is listed as a common side effect affecting up to 1 in 10 people taking Mounjaro. It's usually mild and harmless, resolving within a few days. Severe or persistent bruising should be discussed with your GP.
When should I contact my GP about Mounjaro bruising?
Contact your GP if bruising is severe, spreading beyond the injection site, accompanied by swelling that worsens, signs of infection (warmth, pus, increasing redness), or if bruising doesn't fade after 10–14 days. These could indicate complications requiring medical assessment.
Can I prevent bruising when using the Mounjaro KwikPen?
Yes. Rotate injection sites weekly, inject slowly and smoothly, use room-temperature pens, and apply gentle pressure after injecting. Avoid reusing the same site repeatedly. If you take blood thinners, your GP or prescriber may offer additional guidance on technique.
Is bruising worse with higher Mounjaro doses?
Bruising severity is not directly linked to dose, but to injection technique and individual healing factors. During dose titration, you'll inject more frequently (every week), which increases overall bruising risk simply because you're injecting more often. Proper rotation of sites helps.
What's the difference between normal bruising and a serious injection reaction?
Normal Mounjaro bruising appears as mild purple or yellow discolouration within hours, resolves in 3–7 days, and is painless or minimally tender. Serious reactions include severe swelling, spreading redness, warmth, hardness, pus, or pain lasting days. Seek GP advice if you're concerned about any unusual reaction.
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