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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Blood in Stool with Mounjaro: What You Need to Know
Blood in stool (bloody stools) is not listed as a common side effect of Mounjaro (tirzepatide) in UK clinical trials or prescribing information. However, gastrointestinal side effects are very common with this medication, and blood in stool could indicate a more serious underlying issue. If you notice blood in your stool while taking Mounjaro, it's important to contact your GP immediately for proper assessment. This guide explains what this symptom might mean and when to seek urgent medical attention in the NHS context.
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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.
Understanding Gastrointestinal Side Effects and Blood in Stool
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which affects how your digestive system functions. Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — affecting more than 1 in 10 people in clinical trials. These typically occur during dose escalation and often improve over time as your body adjusts.
Blood in stool can result from several causes, some benign and some requiring urgent attention:
- Haemorrhoids — minor anal tears causing bright red blood, especially with constipation
- Anal fissures — small tears in the anal lining
- Gastrointestinal inflammation — from severe diarrhoea or underlying conditions
- More serious conditions — requiring urgent medical evaluation
Because Mounjaro commonly causes both constipation and diarrhoea, these can contribute to haemorrhoids or anal fissures. However, blood in stool should never be assumed to be a minor side effect — it always warrants GP assessment.
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How It Works
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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When to Contact Your GP and When to Seek Urgent Care
Contact your GP same-day if you notice: blood mixed with stool, persistent bright red bleeding, or blood lasting more than a few days. Your GP can assess whether this is related to Mounjaro or another cause and arrange further investigation if needed.
Seek urgent medical attention (A&E or 999) if you experience: severe rectal bleeding, bloody stools with severe abdominal pain, signs of pancreatitis (severe pain spreading to your back), or fainting or dizziness suggesting significant blood loss. These could indicate a serious complication requiring immediate hospital care, though such cases are rare.
Always inform your prescriber that you're taking Mounjaro when reporting these symptoms — it helps them assess whether the medication needs adjusting or pausing. Never assume gastrointestinal symptoms are simply "normal Mounjaro side effects" without professional evaluation.
- 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 blood in stool a known side effect of Mounjaro?
No, blood in stool is not listed as a common or recognised side effect of Mounjaro in UK clinical trials or MHRA prescribing information. However, Mounjaro commonly causes gastrointestinal side effects like diarrhoea and constipation, which can contribute to secondary causes of bleeding (haemorrhoids or anal fissures). Blood in stool always requires GP assessment.
Could Mounjaro cause internal bleeding?
Internal bleeding is not a recognised side effect of Mounjaro tirzepatide. However, any bleeding from the bowel warrants medical investigation to rule out underlying conditions or complications. Contact your GP if you notice blood in your stool — they can determine the cause and whether it's related to your medication.
What should I do if I see blood when I'm on Mounjaro?
Contact your GP same-day to report it. Describe the colour (bright red vs dark), frequency, and whether it's mixed with stool or on tissue. Your GP will assess whether it's a minor cause like haemorrhoids or something requiring further investigation. Never stop Mounjaro without medical advice — your prescriber needs to know about the symptom first.
Can constipation from Mounjaro cause blood in stool?
Yes, severe constipation can lead to haemorrhoids or anal fissures, causing bright red bleeding. Mounjaro commonly causes constipation, especially when first starting or during dose titration. If you're constipated, increasing fluid intake, fibre, and physical activity can help. Report any bleeding to your GP to confirm it's not a more serious issue.
Should I stop taking Mounjaro if I see blood in my stool?
Do not stop Mounjaro without speaking to your prescriber first. Contact your GP or the service prescribing your Mounjaro UK prescription and describe the bleeding. They'll assess whether the medication needs adjusting, pausing, or continuing. Stopping abruptly without guidance isn't recommended — professional evaluation is essential first.
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