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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What Causes Black Poop on Mounjaro?
Black or dark-coloured stools (melena) can occasionally occur when taking Mounjaro, though it's not listed as a common side effect. This guide explains what black poop means, when you should contact your GP, and how it relates to other gastrointestinal effects of tirzepatide. Understanding Mounjaro side effects helps you recognise what's normal and what requires medical 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.
Black Stools and Mounjaro — When to Worry
Black or very dark stools can indicate bleeding higher in the digestive system, which requires urgent medical evaluation. However, black poop isn't a documented side effect of Mounjaro in clinical trials or the product information leaflet.
Possible causes of dark stools while on Mounjaro include:
- Iron supplements — if you're taking iron tablets, these commonly cause black or very dark stools
- Certain foods — beetroot, liquorice, or very dark foods can darken stool colour
- Medications — some painkillers or other medicines may contribute
- Gastrointestinal bleeding — less common but requires urgent GP assessment
Mounjaro does cause common gastrointestinal side effects like nausea, diarrhoea, and constipation. Constipation in particular can cause changes in stool appearance, though typically not black coloration. If you notice persistently black stools, contact your GP promptly to rule out any underlying issues unrelated to your medication.
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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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Mounjaro Side Effects and Digestive Health
The most common gastrointestinal side effects of tirzepatide include nausea (affecting more than 1 in 10 people), diarrhoea, vomiting, constipation, and stomach pain. These typically occur when starting Mounjaro or during dose increases. Most side effects are temporary and tend to improve as your body adjusts to the medication.
If black stools persist, change suddenly, or are accompanied by abdominal pain, dizziness, or fatigue, seek medical advice from your GP immediately. These could indicate bleeding in the digestive tract, which is unrelated to Mounjaro but requires prompt evaluation. Always report unexpected symptoms to your prescriber rather than assuming they're normal medication side effects.
- 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 black poop a known side effect of Mounjaro?
Black or dark stools are not listed as a known side effect of Mounjaro in clinical trials or the product information. If you experience persistently dark or black stools, it's more likely caused by iron supplements, foods, or other medications — though medical causes should be ruled out by your GP.
What should I do if I have black poop while taking Mounjaro?
Contact your GP to investigate the cause. Bring a list of any other medications or supplements you're taking, especially iron supplements. Your GP can assess whether it's related to your Mounjaro dose, other medications, diet, or requires further investigation.
Are dark stools different from the common side effects of Mounjaro?
Yes. Common Mounjaro side effects are nausea, diarrhoea, constipation, and stomach pain — usually affecting stool consistency rather than colour. Dark stool colour suggests a different cause that should be evaluated medically.
Can constipation from Mounjaro cause black poop?
Constipation from Mounjaro typically causes hard, pale, or normal-coloured stools — not black ones. Black or very dark stools suggest bleeding or another cause unrelated to typical constipation, so medical evaluation is recommended.
When should I seek urgent medical attention for black stools?
Seek urgent help if black stools are accompanied by severe abdominal pain, vomiting blood, dizziness, fainting, or signs of anaemia (extreme fatigue, shortness of breath). These may indicate gastrointestinal bleeding requiring emergency assessment.
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