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

Is black stool a known Mounjaro side effect?

The most important framing here is one that a prescriber would make straight away: black or tarry diarrhoea sits in a completely different clinical category from the gastrointestinal effects Mounjaro is known for. The NHS patient page on tirzepatide lists nausea, vomiting, loose stools, constipation, indigestion and burping as the common GI effects of the medicine. None of those are described as black in colour.

Mounjaro slows gastric emptying, which alters the pace at which food moves through the gut. That change in transit time can shift stool consistency and, sometimes, colour, stools may be paler or slightly different in shade while on tirzepatide. But genuinely black, tarry stool (clinically called melaena) is a sign that blood has passed through and been digested in the upper gut. It carries a distinct smell and a sticky, almost pitch-like consistency that sets it apart from simply dark-coloured stool.

So the short answer is: no, black diarrhoea is not listed in the Mounjaro summary of product characteristics as a side effect you should expect. Its presence alongside tirzepatide treatment is a coincidence that demands investigation, not an assumption that the medicine is responsible in any reassuring way.

What else can cause dark or black stools, and how to tell the difference

Before assuming the worst, it is worth knowing that several common, harmless things turn stools very dark. Iron supplements are the most frequent culprit, they produce greenish-black stools that can alarm patients who weren't warned. Bismuth-containing medicines (found in some indigestion remedies), certain antacids, liquorice and large quantities of very dark foods such as black pudding or activated charcoal products can all produce a similar effect.

A question our prescribers hear most weeks is whether dark stools mean something has gone wrong with the medicine. The honest answer is: it depends what the stool actually looks like. If it is dark because of iron tablets and you feel completely well, that is almost certainly benign. If it is sticky, tar-like, foul-smelling and you feel lightheaded, nauseous in a different way from your usual tirzepatide nausea, or there is any pain, treat it as an emergency.

The safest position is always to get it checked. A GP or urgent care service can ask the right questions quickly, and a simple faecal occult blood test can identify whether blood is present. The broader context of Mounjaro's side-effect profile is worth understanding too, so you have a baseline for what to expect from tirzepatide treatment and can spot deviations early.

When to get medical help without delay

Some symptoms alongside black stools are an immediate 999 or emergency department situation. Call 999 or go straight to A&E if you have black or tarry diarrhoea combined with any of the following: sudden dizziness or feeling faint, a rapid or irregular heartbeat, cold or clammy skin, vomiting blood or anything that looks like coffee grounds, severe abdominal pain, or a sharp drop in how alert you feel. These point to significant active bleeding.

Contact your GP or an urgent care service the same day (rather than waiting) if stools are black and tarry but you feel broadly okay, or if the change has persisted for more than one stool. Do not take a wait-and-see approach simply because you are on a weight-loss medicine and assume the medicine is the cause.

Separately, Mounjaro carries a known association with pancreatitis as an infrequent but serious risk. The MHRA highlighted this in a Drug Safety Update, noting that severe, persistent stomach pain that radiates to the back, with or without vomiting, requires urgent medical attention. That symptom is distinct from black stool, but both deserve the same seriousness. You can read more about the full range of bowel changes that can follow Mounjaro, including what typically resolves on its own. If you have concerns you'd like to discuss with a clinician, our team is available seven days a week.

Reporting your experience and getting ongoing support

If you believe a side effect is connected to Mounjaro (including unexpected changes to stool colour or consistency) you have the right to report it directly to the MHRA. The Yellow Card scheme accepts reports from patients, not just clinicians, and that reporting genuinely shapes how medicines are monitored after licensing. It is particularly encouraged for Black Triangle medicines; Mounjaro carries the ▼ symbol, meaning the MHRA is actively gathering post-market safety data.

If you are already a patient and something unexpected has happened, contacting your prescriber is the right first step before adjusting or stopping your dose. Changes to your treatment schedule should always be made with clinical input. A broader overview of diarrhoea as a Mounjaro side effect covers the more typical bowel changes patients experience, which are different in nature from what is described on this page, and patients who want to understand how Mounjaro diarrhea tends to present and progress will find that detail useful when judging whether their own experience is within the expected range. If you are also trying to understand what causes diarrhea from Mounjaro at a physiological level, that page explains the mechanisms behind the gut changes tirzepatide can trigger. Separately, anyone who has experienced serious complications and wants to understand their position may find it helpful to read about the Mounjaro side effects lawsuit landscape, which sets out what has been reported and pursued in legal proceedings to date. For anyone still at the stage of deciding whether tirzepatide is right for them, our prescribers cover the full side-effect picture and your individual health history as part of the free initial consultation, there is no commitment involved.