Dark Stool on Wegovy: What's Normal, What Isn't, and When to Call a Doctor

Step 1, Work out whether diet or supplements are the most likely cause

Before drawing conclusions, it is worth spending a minute running through what you have eaten or taken in the past 24–48 hours. Certain foods produce convincingly dark stool with no medical significance at all. Liquorice, blueberries, beetroot, dark leafy greens consumed in large quantities, and foods with artificial black or dark food colouring are among the most common culprits. Iron supplements (often recommended alongside GLP-1 treatment to offset reduced dietary intake) are a frequent cause of very dark, sometimes almost black stool, as is bismuth-containing medication.

Wegovy itself slows gastric emptying, which means food spends longer in the digestive tract. That slower transit can alter how stool looks by the time it passes, particularly if your diet has changed alongside starting treatment. Many people on semaglutide reduce their portion sizes considerably, increasing the relative proportion of iron-rich or pigment-heavy foods in their diet without realising it. The broader range of bowel changes semaglutide can cause is worth understanding, because stool colour rarely changes in isolation.

If you can point confidently to a dietary reason, monitor for 24–48 hours and see whether the colour normalises once you adjust what you have eaten. That simple check takes under a minute and can save a great deal of unnecessary worry.

Step 2, Recognise the signs that need medical attention today

The clinical concern with dark stool is a specific pattern: sticky, tar-like, very dark (often described as coal-black), and noticeably foul-smelling. This appearance (known medically as melaena) suggests that blood has passed through the upper digestive tract and been digested, changing its colour in the process. It is a different finding from stool that is simply darker than usual or slightly greenish-brown.

You should seek same-day medical help if your stool has a tar-like consistency, if the darkness has persisted for more than 48 hours with no dietary explanation, or if it is accompanied by stomach pain, vomiting (especially if the vomit looks dark or like coffee grounds), dizziness, or unexpected weakness. These combinations can indicate upper GI bleeding, which requires urgent assessment.

It is also worth reading the guidance on the possibility of blood in stool on Wegovy and, separately, on bloody stool more broadly, because the visual distinction between digested blood and fresh blood matters clinically. The NHS tirzepatide and semaglutide medicines pages both advise contacting a healthcare professional promptly if you experience any unexpected gastrointestinal symptoms that concern you, that guidance applies here.

Step 3, Understand the wider GI picture of Wegovy treatment

Gastrointestinal effects are the most commonly reported side effects of semaglutide. In the STEP 1 trial, published in the New England Journal of Medicine, GI events were the most frequent reason for discontinuation in the active treatment group, though most participants tolerated the medicine. Nausea, constipation, diarrhoea, indigestion, and changes in bowel habit are all documented. Stool colour or consistency changes can sit within this picture without signifying anything beyond the medicine's action on gut motility.

That said, the MHRA Drug Safety Updates for GLP-1 medicines are worth knowing about: regulators have highlighted acute pancreatitis as an infrequent but serious possibility, with severe stomach pain radiating to the back as the key symptom. Pancreatitis is not a cause of dark stool directly, but severe abdominal symptoms appearing alongside stool changes should always be treated as potentially serious. The less-discussed side effects of Wegovy cover this broader safety picture in more detail.

Changes to urine colour can occur alongside GI symptoms too, particularly if dehydration develops from vomiting or diarrhoea; the page on dark urine on Wegovy explains what to watch for. Keeping well hydrated is one of the more practical things you can do to reduce GI side effect severity during titration.

Step 4, What to do next if you are unsure

If you cannot rule out a dietary cause, or if the stool change has lasted beyond 48 hours, contact your prescriber or GP directly. Do not wait for your next scheduled review. Patients receiving treatment through nume can reach our aftercare team seven days a week through the contact page, a real clinician reviews concerns, not an automated response system.

For anyone researching Wegovy before starting treatment, understanding the full side-effect profile forms part of the pre-treatment conversation. The Wegovy treatment overview explains how semaglutide works and what the clinical assessment involves, and the semaglutide information page covers mechanism and evidence in more depth. Wegovy is a prescription-only medicine; a prescriber assesses your individual suitability, including any personal history that might affect how you tolerate it.

If you are considering starting treatment and want to understand whether it is clinically appropriate for you, a free consultation with our prescribers is the right first step. Check your eligibility, there is no obligation, and your answers are reviewed by a GPhC-registered Independent Prescriber on the same day.