Does Wegovy Cause Black Stool? Separating Fact from Concern

The misconception: Wegovy causes black stool as a side effect

This is the question worth correcting first, because a lot of people searching for an answer have already half-convinced themselves the answer is yes. It is not. The UK Summary of Product Characteristics for semaglutide (the document that lists every significant adverse event observed across Wegovy's clinical programme) does not include black stool or dark stool in its side-effect profile. The STEP 1 trial, which ran for 68 weeks and forms the backbone of Wegovy's evidence base, reported no pattern of melanotic stool among participants.

What the trials did show clearly is that semaglutide produces a GI-heavy side-effect profile: roughly half of participants experienced nausea at some point, diarrhoea and constipation were both common, and indigestion was regularly reported. These effects cluster around the first weeks of treatment and after each dose step. Stools can change in consistency or frequency for these reasons. But a change in colour to black or tarry is not part of that picture, and conflating the two can both unnecessarily alarm people and, more worryingly, lead others to dismiss a genuinely important symptom as a routine drug side effect. You can read about the full range of effects that have been linked to Wegovy for broader context on what is and is not on the label.

The NHS medicines page for semaglutide provides a plain-English summary of the side effects observed in clinical practice, and black stool does not feature there either.

What black stool actually means, and why it matters regardless of your medication

Stools that are black, sticky and tarry (medically called melaena) are a recognised sign of bleeding somewhere in the upper gastrointestinal tract: the oesophagus, stomach or upper small intestine. The dark colour comes from digested blood, haemoglobin broken down by stomach acid as it travels through the gut. This can result from a peptic ulcer, a Mallory-Weiss tear (sometimes caused by repeated vomiting), oesophageal varices or other conditions that exist independently of whatever medicine someone is taking.

Here is the connection worth understanding: Wegovy can cause significant nausea and vomiting, especially early in treatment. Repeated forceful vomiting can, in rare cases, cause small tears in the oesophageal lining. That is not a mechanism the medicine directly produces, it is a secondary consequence of a side effect. It is also not the same as saying Wegovy causes black stool; it means that if vomiting is severe and prolonged, other problems can follow. Anyone experiencing any sign of blood in the stool while on semaglutide should get that assessed promptly rather than attributing it to the drug and waiting.

The MHRA's January 2026 Drug Safety Update for GLP-1 receptor agonists highlights acute pancreatitis as a serious but infrequent risk with this class, relevant here because severe upper abdominal pain can accompany both pancreatitis and GI bleeding. These are separate conditions with overlapping warning signs, and self-diagnosis at home is the wrong approach to either.

When to get medical help and what to tell your doctor

If your stool is black and tarry rather than just darker than usual, seek same-day medical advice. That guidance applies whether or not you are on Wegovy. Other signs that need prompt attention include vomiting blood or material that looks like coffee grounds, severe abdominal pain that radiates to your back, and feeling faint or unusually weak alongside a bowel change.

When you contact your GP or call 111, tell them you are taking semaglutide and at what dose. This matters because GLP-1 medicines slow gastric emptying, which can affect how other conditions present and may influence which investigations are appropriate. Your prescriber at nume is also available through our aftercare line seven days a week, but if the symptom appears urgent, go to your GP or an urgent-care service first rather than waiting for an online response.

Not everything dark in the bowl is concerning. Iron supplements, bismuth compounds, liquorice, red wine and foods high in dark pigments can all cause very dark stool without any bleeding. If you have started iron tablets recently or eaten a lot of black pudding, that is worth mentioning to a clinician alongside any Wegovy history, context helps rule things out faster. It is also worth knowing that other unexpected symptoms during semaglutide treatment often have similarly prosaic explanations, but checking them is always the right call.

Getting the right clinical review before and during treatment

If you have a history of peptic ulcers, gastro-oesophageal reflux disease or any condition affecting the upper GI tract, these are exactly the things a prescriber should know about before Wegovy is started. The consultation process at nume includes a full clinical assessment, a GPhC-registered prescriber reads your health history, not a checklist algorithm. Anyone with active GI bleeding or a recent significant GI event would not be an appropriate candidate, and a clinician is best placed to make that call individually.

For anyone already on Wegovy who notices a stool change: report it. Side effects can be flagged to the MHRA's Yellow Card scheme by patients directly, which contributes to ongoing post-market surveillance of medicines like semaglutide. It is genuinely useful, not just a formality. If the change turns out to be dietary or temporary, no harm done. If it is something that needs investigating, you will be glad you checked early. Our Wegovy overview covers the broader clinical picture for anyone researching the medicine from the beginning, and if you are currently comparing prices or timing a purchase around seasonal promotions, our Wegovy Black Friday page has the details you need. And if you are still weighing up whether a clinician-led weight-management service is the right route, our clinical team page explains who reviews every consultation.

If you are ready to start a conversation with a prescriber, you can speak to our prescribers through a free consultation, same day where the clinical picture allows.