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 And Bruising Compared To Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections given once weekly, and both carry a risk of injection site reactions including bruising. While clinical trials show these reactions are generally mild and temporary, understanding how each medication compares can help you discuss options with your prescriber. This guide compares bruising rates, mechanisms, and what to expect during treatment with either medication.
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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.
Bruising Risk: Clinical Trial Data And Mechanisms
Injection site bruising is classified as a common side effect for both medications, affecting up to 1 in 10 people in clinical trials. However, the frequency and severity differ slightly between Mounjaro and Wegovy.
Mounjaro (tirzepatide) uses a dual GIP and GLP-1 receptor agonist mechanism — a different approach from Wegovy — and bruising occurred in approximately 8–10% of participants in the SURMOUNT trials. The KwikPen delivery system may also influence bruising risk due to needle gauge and injection pressure.
Wegovy (semaglutide) is a GLP-1-only agonist, and bruising appeared in similar rates (around 7–9%) during the STEP clinical trials. The pre-filled auto-injection pen design is slightly different from Mounjaro's KwikPen.
Both medications may increase bruising if combined with blood thinners or anticoagulants — important to discuss with your GP before starting either injection.
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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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Individual Suitability And Prescriber Assessment
Whether you receive Mounjaro or Wegovy depends on individual clinical assessment, not bruising risk alone. Your prescriber will assess your full medical history, other medications, skin condition, and treatment goals. Some patients tolerate one medication better than the other, and switching is possible if side effects become problematic.
Practical steps to minimise bruising with either injection include rotating injection sites weekly (stomach, thigh, or upper arm), allowing pens to reach room temperature before injecting, and using proper injection technique. Most bruising resolves within 1–2 weeks and becomes less frequent as your body adapts. If bruising is severe or persistent, contact your GP — they may adjust your approach or consider alternative options.
- 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 Mounjaro more likely to cause bruising than Wegovy?
Clinical trials show similar bruising rates for both medications (approximately 8–10% for Mounjaro, 7–9% for Wegovy). Individual experiences vary significantly. Injection technique, site rotation, and skin sensitivity matter more than the medication itself. Your prescriber can advise based on your specific risk factors.
Can I reduce bruising with either injection?
Yes. Rotate injection sites each week between stomach, thigh, and upper arm. Allow the pen to reach room temperature before injecting. Use proper injection technique and avoid injecting into areas already bruised. Most bruising is mild and resolves within 1–2 weeks as your body adapts to the injections.
What if I'm taking blood thinners — which medication is safer?
Both Mounjaro and Wegovy may increase bruising risk if you take anticoagulants or blood thinners like warfarin or aspirin. This is not a contraindication, but your GP must know about all medications. They'll assess whether either medication is suitable and monitor you closely during the dose titration phase.
How do Mounjaro and Wegovy injection pens differ?
Mounjaro uses the KwikPen (4 doses per pen) and Wegovy uses a pre-filled auto-injection pen. Both are once-weekly injections. The different pen designs may affect injection comfort slightly, but clinical bruising rates are comparable. Your prescriber will show you how to use whichever you're prescribed.
Should bruising be a reason to choose one medication over the other?
Bruising alone is rarely the deciding factor — your prescriber considers weight loss effectiveness, other side effects, existing health conditions, and drug interactions. If bruising becomes problematic with one medication, discuss switching with your GP. Both are effective, and individual tolerability varies widely.
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