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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Can Mounjaro Cause Blood In Stool?
Blood in stool is not listed as a common side effect of Mounjaro (tirzepatide) in UK clinical trials, though severe gastrointestinal complications including pancreatitis and gallbladder issues are rare but serious concerns. If you notice blood in your stool while taking Mounjaro or any weight loss injection, you should contact your GP immediately for medical evaluation. This guide compares Mounjaro and Wegovy gastrointestinal side effects and explains what requires urgent attention.
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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.
Mounjaro vs Wegovy: Gastrointestinal Side Effects
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 based weight loss injections licensed in the UK, and both share similar gastrointestinal side effects because they work on the same hormone pathway. The most common GI side effects reported in clinical trials include nausea, vomiting, diarrhoea, and constipation — affecting more than 1 in 10 users.
Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy activates GLP-1 only. This difference may affect symptom intensity for some users, though individual responses vary widely. Neither medication lists blood in stool as a typical side effect in SURMOUNT or STEP trial data. However, severe gastrointestinal events — including pancreatitis, which can cause bloody stools in rare cases — are possible with both medications.
Your prescriber will assess whether Mounjaro, Wegovy, or another approach suits your individual health profile. Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication.
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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.
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When Blood In Stool Requires Urgent Care
If you develop blood in your stool while taking Mounjaro via its KwikPen delivery device, seek medical attention immediately. This could indicate several conditions unrelated to the medication itself — including haemorrhoids, anal fissures, or inflammatory bowel disease — or it could signal a rare serious complication like pancreatitis.
Seek urgent medical help if you experience:
- Severe, persistent stomach pain spreading to the back (possible pancreatitis)
- Vomiting blood or material that looks like coffee grounds
- Black, tarry stools (indicating upper GI bleeding)
- Bright red blood in stool with severe pain
- Bloody stools alongside other symptoms like fever or dizziness
Do not assume it is a minor issue. Your GP can investigate the cause and advise whether you should continue your medication. Mounjaro eligibility and suitability is always a prescriber decision based on your full health picture.
- 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?
Blood in stool is not listed as a common or expected side effect in SURMOUNT clinical trial data. However, rare serious gastrointestinal complications including pancreatitis can occur, which may present with bloody stools. Any blood in stool requires medical evaluation from your GP.
Does Wegovy cause blood in stool?
Like Mounjaro, bloody stools are not a typical Wegovy side effect reported in STEP trials. Both medications can cause nausea, vomiting, and diarrhoea. Severe GI complications are rare but possible. Contact your GP immediately if you notice blood.
What is the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. This may cause different symptom patterns for some users, though both share similar common side effects like nausea and constipation. Your prescriber decides which is suitable.
What should I do if I have bloody stools on Mounjaro?
Contact your GP immediately. Do not assume it is medication-related — it could indicate haemorrhoids, fissures, or a serious condition. Your GP can investigate and advise whether to continue Mounjaro or switch medications. Never stop suddenly without guidance.
Can Mounjaro cause pancreatitis with bloody stools?
Pancreatitis is a rare but serious possible complication of GLP-1 medications including Mounjaro. It typically presents with severe stomach pain spreading to the back, often with vomiting, not necessarily bloody stools. Seek emergency care for severe abdominal pain.
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