Mounjaro and Black Stools: What Your Body Might Be Telling You

You've looked in the bowl and something looks wrong, here's how to think about it

Picture this: you're a few weeks into Mounjaro, you've been managing the nausea reasonably well, and then one morning something about your stool looks much darker than normal. Before anything else, run through what you've eaten and taken in the last 48 hours. Iron tablets, blueberries in large quantities, black liquorice, dark leafy greens cooked down, and red wine can all produce surprisingly dark stools with no medical significance. If you started an iron supplement around the same time as Mounjaro, that is the most common explanation our prescribers hear.

Mounjaro itself slows gastric emptying, that is part of how it works. Slower transit through the gut gives bacteria more time to act on the contents, and stools may appear darker or change in consistency, especially in the early weeks of treatment. Loose stools and changes in frequency are well-documented; you can read more about how loose stools present on Mounjaro and whether they tend to settle. None of this causes the kind of darkness that signals bleeding, but if you are unsure, the distinction below matters a great deal.

The NHS medicines page for tirzepatide covers the recognised side-effect profile in full. Dark or black stools do not appear there as an expected effect, which is exactly why you should not dismiss them as "just the medication" without thinking it through carefully.

What melaena looks like, and why it is different from ordinary dark stools

True melaena is distinctive. Stools are not just dark, they are jet black, sticky like tar or wet tarmac, and carry a particularly foul smell. That smell comes from digested blood that has passed through the stomach and upper intestine. Even a relatively small bleed higher up in the digestive tract can produce this appearance, because stomach acid and digestive enzymes break down the haemoglobin in blood and turn it black by the time it exits.

This is categorically different from the dull, greenish-brown darkness that iron supplements or certain foods produce. If you press a piece of toilet paper gently against the stool and it smears black and sticks, that is a warning sign. If you are also experiencing severe stomach pain (particularly pain that radiates through to your back) dizziness, or you have vomited anything that looks like coffee grounds, call 999 or go to A&E without delay. The MHRA's January 2026 Drug Safety Update on GLP-1 medicines flagged acute pancreatitis as a serious infrequent risk, and gastrointestinal bleeding, though not unique to Mounjaro, warrants the same urgency of response.

For more on blood appearing in stools specifically, our page on Mounjaro and blood in stool goes into the distinction between bright red blood and the darker presentation described here. The pattern matters for what happens next.

Darker stools that are not melaena: what Mounjaro's GI effects actually look like

Tirzepatide's most common side effects are gastrointestinal: nausea, vomiting, diarrhoea, constipation, indigestion. The Mounjaro overview covers this in context. These effects tend to be most noticeable after starting treatment or after a dose increase, and they usually ease within one to two weeks as the body adjusts. Constipation can make stools darker and harder simply because they are spending longer in the colon. Dehydration from vomiting or diarrhoea concentrates waste and can deepen colour. Neither of these is melaena, and neither needs emergency treatment, though persistent vomiting or diarrhoea that is leaving you unable to keep fluids down does warrant contacting your GP or prescriber promptly.

If you are considering how tirzepatide works mechanically as a dual GIP and GLP-1 receptor agonist, the gut-slowing effect is a useful backdrop here. It is not a bug but a feature of the medicine's appetite and blood-sugar regulation mechanism, and understanding it can make the early weeks less worrying. The page specifically on dark stools and Mounjaro covers the shade spectrum in more detail if you want to compare what you're seeing with documented presentations.

One practical note: some people keep their Mounjaro pen in the fridge door next to everyday items and barely give the injection a second thought once the routine is set, but any new symptom that feels out of the ordinary is worth pausing on. If you are also keeping an eye on pricing and timing your treatment, our Mounjaro Black Friday page rounds up the current deals worth knowing about. A call to your prescriber costs nothing and takes five minutes.

When to get help, and what to do through nume's aftercare

The threshold for seeking urgent help is straightforward: if stools are black, tarry and foul-smelling (particularly alongside stomach pain, dizziness, or vomiting) treat it as a medical emergency and call 999 or attend A&E. Do not wait to see if it passes. If the symptom is less dramatic but you are worried, contact your GP or prescriber. Side effects can also be reported to the MHRA directly via the Yellow Card scheme, which helps build the safety evidence base for medicines still under monitoring.

For those on Mounjaro through nume, our prescribers are available seven days a week through aftercare. You can reach the team via our contact page. This is not a service for emergencies (for anything urgent, call 999 or NHS 111) but if you want a clinical opinion on a symptom that is worrying you, our team is there. The FAQs page also covers a range of common questions about treatment. If you are not yet a patient and want to understand what treatment through a regulated service looks like, check your eligibility with a free consultation reviewed by a GPhC-registered prescriber.