Dark Stools on Mounjaro: What's Causing Them and What to Do

Is it the Mounjaro itself that's turning stools dark?

Tirzepatide slows the movement of food through your digestive system, a well-documented effect sometimes called delayed gastric emptying. When digestion moves more slowly, stools can become darker and more concentrated in colour than usual. This happens because the gut has more time to absorb water and break down bile pigments before they pass through. It can be surprising the first time, and a common assumption is that something must be bleeding. Usually, it isn't.

Diet shifts also play a role. Many people on Mounjaro eat less overall and sometimes change what they eat (more protein, less processed food) and certain foods darken stools reliably: dark leafy greens, beetroot, black beans, liquorice, and blueberries among them. Iron supplements, which some people take alongside weight-management treatment, will turn stools very dark or even near-black on their own. If you've recently started an iron supplement or changed your diet noticeably, that's the first thing to consider.

The NHS tirzepatide patient information page lists gastrointestinal effects (including changes to bowel habits) among the more common experiences during treatment, particularly after starting or after a dose change. These effects tend to settle within a couple of weeks as the body adjusts. For context on the broader digestive picture, our guide to loose stools on Mounjaro covers how tirzepatide affects gut transit more generally.

When do dark stools point to something that needs urgent attention?

There is an important distinction between dark stools and what clinicians call melaena. Melaena is jet-black, sticky, and has a distinctive foul smell, somewhat like tar. It is caused by digested blood that has come from somewhere in the upper gastrointestinal tract (the stomach or upper small intestine) and it is a medical emergency. If you see stools matching that description, seek urgent care the same day. Do not wait to see if it passes.

Other symptoms that should prompt you to seek help quickly alongside any stool change: vomiting blood or material that looks like coffee grounds, severe or persistent stomach pain that radiates towards the back, light-headedness, or a noticeably fast heartbeat. The MHRA's Yellow Card scheme also accepts reports of suspected side effects from patients directly, and using it helps the regulator monitor GLP-1 medicines in real-world use.

Pancreatitis (a known but uncommon serious effect with GLP-1 medicines) can sometimes produce dark stools alongside severe abdominal pain. It is not something to diagnose yourself, but it is one reason why persistent stomach pain should never be ignored while on this type of treatment. Our page on blood in stool on Mounjaro covers the bleeding-specific signs in more detail.

What should you actually do if your stools have darkened since starting Mounjaro?

Start with a practical check: did you start an iron supplement recently, or eat a meal heavy in dark-coloured foods in the last day or two? Has the colour returned to normal within 48 hours? If the answer to either of those is yes, the cause is almost certainly benign and worth monitoring rather than panicking about.

If the discolouration is persistent (more than two or three days) or the stool has that tarry quality rather than simply being a darker shade of brown, contact your prescriber or GP. Do not stop Mounjaro without clinical guidance; stopping abruptly is rarely the right answer and is a decision for your prescriber, who knows your full health picture.

Changes that are murkier (an unusual texture alongside colour, or a greasy look) might reflect how fat is processed and absorbed differently on tirzepatide, something worth understanding if you are noticing changes to stool appearance. The page on mucus in stool on Mounjaro and the one on loose stools both address texture-related concerns that often come up alongside colour questions. For a fuller overview of what tirzepatide is and how it works, our tirzepatide information page is a good starting point.

If you are considering starting Mounjaro and want to understand the full side-effect profile before you commit, our free consultation is the right place to ask. A GPhC-registered prescriber reviews every application the same day and can answer questions specific to your health history.