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 Dark Stools — What You Need to Know
Dark stools can be a concerning side effect when taking Mounjaro (tirzepatide), though they are not among the most commonly reported side effects. If you are experiencing dark or tarry stools whilst on this prescription weight management injection, it is important to understand what might be causing them and when you should contact your GP. This guide explains the possible causes, NHS guidance, and when to seek 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.
What Causes Dark Stools on Mounjaro?
Dark stools whilst taking Mounjaro (tirzepatide) can have several causes. The most common reason is related to how Mounjaro affects your digestive system. This injection slows gastric emptying—the speed at which food moves from your stomach into your small intestine. When food moves more slowly through your digestive tract, it spends more time in your stomach and intestines, which can affect stool appearance and colour.
Dark or tarry stools may also indicate:
- Dehydration—Mounjaro commonly causes nausea and vomiting, which can lead to reduced fluid intake
- Gastrointestinal bleeding (less common but serious)—bleeding in the upper digestive tract can cause dark, tarry stools
- Dietary changes—eating less food or different foods when taking Mounjaro can affect stool colour
- Iron supplements or iron-rich foods—these naturally cause darker stools
- Other medications you may be taking alongside the injection
The key is understanding whether the dark stools are a benign side effect or a sign of something requiring urgent medical attention.
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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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When to Contact Your GP About Dark Stools
You should contact your GP promptly if you experience dark stools alongside other symptoms. These warning signs require medical evaluation: stomach pain or persistent abdominal discomfort, vomiting blood or material that looks like coffee grounds, dizziness or lightheadedness, unexplained fatigue, or prolonged dark stools lasting more than a few days without improvement.
Call NHS 111 for urgent advice outside surgery hours, or attend A&E if you vomit blood or experience severe stomach pain. Your GP can assess whether this is a benign side effect or a sign of gastrointestinal bleeding. Be clear about when the dark stools started in relation to starting or increasing your Mounjaro dose, and mention any other symptoms. Your prescriber may adjust your dose or recommend stopping treatment temporarily whilst investigations are carried out.
- 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.
Are dark stools a common side effect of Mounjaro?
Dark stools are not listed as a very common side effect in the clinical trial data. However, they can occur as an indirect effect of how Mounjaro slows digestion, or potentially as a sign of gastrointestinal bleeding. If you experience them, contact your GP to rule out bleeding.
Should I stop taking Mounjaro if my stools are dark?
Do not stop taking your Mounjaro without speaking to your prescriber first. If the dark stools are simply due to slowed digestion or dietary changes, stopping may not be necessary. However, if they are accompanied by other warning symptoms, contact your GP immediately for guidance.
Can dark stools mean I'm bleeding internally?
Dark, tarry stools can occasionally indicate bleeding in the upper digestive tract, which requires urgent medical attention. This is why it is important to contact your GP or NHS 111 if you experience dark stools, especially alongside stomach pain, vomiting blood, or dizziness.
What is the difference between dark stools and normal stool colour changes?
Normal stool colour can vary based on diet and hydration. Dark stools related to Mounjaro are typically very dark brown or black, tarry-looking, and may be accompanied by other symptoms. If you are unsure, describe them to your GP or use NHS 111 for advice.
Can I reduce the risk of dark stools whilst on Mounjaro?
Stay well hydrated, eat small frequent meals when you can tolerate them, and avoid foods that might naturally cause darker stools if you are concerned. If nausea is causing you to skip meals or drink less, discuss this with your prescriber—they can advise whether dose adjustment might help.
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