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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Dark Stools on Mounjaro — What You Need to Know
Dark stools can occur while taking Mounjaro (tirzepatide), though they are not listed as a common side effect in the official UK prescribing information. Dark stools may indicate a change in digestion or, in rare cases, the presence of blood from the gastrointestinal tract. This guide explains what dark stools mean, when to be concerned, and when to contact your GP. If you're considering Mounjaro or tracking your health journey, our free GLP-1 weight loss calculator can help you understand the potential effects of this prescription-only medication alongside realistic expectations for weight management.
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Calculate Your BMI Instantly
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.
What Causes Dark Stools on Mounjaro?
Dark stools can have several causes when taking Mounjaro (tirzepatide). Black or very dark stools may indicate the presence of blood in the upper gastrointestinal tract, which requires immediate medical attention. However, dark stools can also result from:
- Dietary factors — foods like beetroot, prunes, or iron supplements can naturally darken stool colour
- Slowed gastric emptying — Mounjaro slows how quickly food moves through your stomach, which can affect stool appearance and consistency
- Changes in bile production — altered digestion may change stool pigmentation
- Dehydration — common with gastrointestinal side effects like nausea or vomiting, which can concentrate stool and darken it
- Medication interactions — other medicines you take may contribute to stool colour changes
In clinical trials of Mounjaro (tirzepatide), dark stools were not reported as a common side effect, but gastrointestinal effects like constipation, diarrhoea, and stomach pain were very common. Dark stools are less frequent but still possible.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When to Contact Your GP About Dark Stools
Contact your GP urgently if you experience:
- Dark or black stools accompanied by stomach pain, nausea, or vomiting
- Persistent dark stools lasting more than a few days
- Signs of anaemia — fatigue, dizziness, shortness of breath, or pale skin
- Stools that look like tar or coffee grounds
- Any concern that you may be bleeding internally
These symptoms could indicate a more serious gastrointestinal problem that requires investigation. If you're on Mounjaro and notice dark stools, keep a record of when they occur and any other symptoms. This information helps your GP assess whether Mounjaro is suitable for you or whether your dose should be adjusted. Always inform your prescriber of any unexplained changes in stool colour or consistency.
- 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 dark stools a known side effect of Mounjaro in the UK?
Dark stools are not listed as a common side effect in the official UK prescribing information for Mounjaro. However, gastrointestinal effects are very common with tirzepatide, and changes in digestion may occasionally result in altered stool appearance. If you experience dark stools, contact your GP to rule out other causes.
Could dark stools mean I'm bleeding while taking Mounjaro?
Dark or black stools (especially if they resemble tar or coffee grounds) may indicate upper gastrointestinal bleeding, which requires urgent medical attention. This is rare but serious. Contact your GP or call 111 immediately if you suspect internal bleeding, especially if accompanied by stomach pain or vomiting.
How long should I wait before contacting my GP about dark stools?
Contact your GP if dark stools persist for more than a few days, or immediately if accompanied by stomach pain, nausea, fatigue, or dizziness. Do not wait — gastrointestinal bleeding can be serious. Your GP may need to examine you or refer you for further investigation.
Can diet cause dark stools while taking Mounjaro?
Yes, diet can contribute to darker stools. Foods high in iron (red meat, spinach, fortified cereals), beetroot, prunes, or iron supplements can all darken stool colour naturally. However, if the darkness is unusual or persistent, mention it to your GP, especially alongside other symptoms.
Should I stop taking Mounjaro if I notice dark stools?
Do not stop taking Mounjaro without advice from your prescriber. Contact your GP first to determine the cause. Your prescriber may adjust your dose, investigate further, or continue Mounjaro depending on the findings. Never stop a prescription medication without medical guidance.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.