Black Poop on Mounjaro: What It Means and When to Act

Not every instance of dark-coloured stool signals a medical emergency, but it is important to understand the range of possible causes before drawing any conclusions.

Food and supplement-related causes

Certain foods and supplements can temporarily darken stools without any harm being done. Iron supplements are a well-known cause of black or very dark poo, as is eating large amounts of dark foods such as liquorice, blueberries, or black beans. If you have recently changed your supplement routine — which is common when starting a GLP-1 medicine — this is worth considering.

When black stools may signal something more serious

A tarry, sticky consistency and an unusually strong or unpleasant smell are characteristics that can suggest digested blood, sometimes called melaena. This can indicate bleeding in the stomach or upper small intestine. GLP-1 receptor agonists like tirzepatide and semaglutide slow gastric emptying, which means digestive contents spend longer in the stomach — this does not directly cause bleeding, but if you already have an underlying condition such as a peptic ulcer, symptoms may become more noticeable.

Black poop that is tarry and foul-smelling is a recognised reason to seek medical attention without delay. According to NHS guidance, black or tarry stools that are not explained by food or iron supplements should always be assessed by a healthcare professional. Your prescriber or doctor can help determine the cause and advise on next steps.

If you are also experiencing symptoms such as abdominal pain, dizziness, nausea, or vomiting alongside black stools, contact a healthcare professional promptly — do not wait for a routine appointment.

While you are looking into gut-related changes on your GLP-1 journey, you may also find it useful to read about green stools and other bowel changes on Mounjaro, which have their own set of causes and context. For a broader overview of the medicine itself, the complete Mounjaro guide covers common experiences reported by people taking tirzepatide.

The most useful thing you can do when you notice any unusual symptom — including changes in stool colour — is to record it clearly so you can discuss it with your healthcare professional.

GLP Trackr's side-effect and symptom tracker lets you log exactly what you observed, when it started, how long it lasted, and any other factors that were in play that day, such as supplements you took or foods you ate. Having that timeline written down means you can give your prescriber or doctor a precise picture rather than relying on memory during an appointment — and it makes it much easier for them to assess whether a pattern is emerging.

Beyond symptom logging, the daily check-in feature captures how you are feeling overall each morning, including energy levels and any digestive discomfort. Over time, this builds a personal record of your journey that can be genuinely useful in prescriber conversations. You can even export a summary report to bring to your next appointment.

Important: GLP Trackr is a tracking and journaling tool — it is not a substitute for medical advice. If you are seeing black, tarry stools and are unsure of the cause, contact your prescriber or doctor promptly. Any decision about your medication should be made with your healthcare professional, never on your own. If you ever experience a side effect you want to formally report, your country will have a national pharmacovigilance scheme for this purpose (such as the UK's Yellow Card scheme) — your prescriber or pharmacist can advise you on the appropriate route.

It is also worth being aware of how important it is to source your medication safely and legally — if you have any concerns in that area, this overview of the risks of unregulated Mounjaro sources is worth reading.

Ready to keep a clear record of how your body responds throughout your GLP-1 journey? GLP Trackr is available free on the App Store and Google Play, with a 7-day trial to get started — download it and let the symptom tracker do the work.