Ozempic (semaglutide) is a widely prescribed GLP-1 receptor agonist for managing type 2 diabetes in the UK. Whilst gastrointestinal side effects such as nausea and diarrhoea are well documented, patients occasionally report changes in stool appearance, prompting questions about whether Ozempic causes dark stools. Dark stools are not listed as a recognised side effect in official prescribing information, yet understanding when stool discolouration warrants medical attention remains essential. This article examines the evidence, distinguishes benign causes from serious gastrointestinal bleeding, and provides guidance on managing digestive symptoms whilst taking Ozempic.

Understanding Ozempic and Its Common Side Effects

Ozempic (semaglutide) is a glucagon-like peptide-1 (GLP-1) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus. Administered as a once-weekly subcutaneous injection, it works by mimicking the action of the naturally occurring hormone GLP-1, which stimulates insulin secretion in response to elevated blood glucose levels, suppresses glucagon release, and slows gastric emptying. This mechanism helps improve glycaemic control in people with type 2 diabetes.

As with any medication, Ozempic is associated with a range of side effects, the majority of which affect the gastrointestinal system. The most commonly reported adverse effects include nausea, vomiting, diarrhoea, abdominal pain, constipation, and decreased appetite. These symptoms typically emerge during the initial weeks of treatment or following dose escalation, and often diminish as the body adapts to the medication. According to the Summary of Product Characteristics (SmPC) approved by the Medicines and Healthcare products Regulatory Agency (MHRA), gastrointestinal disturbances occur in a significant proportion of patients, with nausea affecting up to 20% of users.

Other notable side effects include injection site reactions and hypoglycaemia—particularly when Ozempic is used in combination with insulin or sulphonylureas. Rare but serious adverse effects such as pancreatitis, diabetic retinopathy complications, and acute kidney injury have also been documented. Patients commencing Ozempic should be counselled about these potential effects and advised to report any persistent or concerning symptoms to their healthcare provider. Understanding the expected side effect profile is essential for distinguishing between common, self-limiting reactions and those requiring urgent medical evaluation.

Does Ozempic Cause Dark Stools?

Dark stools are not listed as a recognised or common side effect of Ozempic in the official prescribing information approved by the MHRA or the European Medicines Agency (EMA). There is no established pharmacological mechanism by which semaglutide would directly cause stool discolouration. However, patients taking Ozempic may experience changes in bowel habits—including constipation or diarrhoea—which can occasionally influence stool appearance.

It is important to distinguish between different types of dark stools. Melaena—black, tarry, and foul-smelling stools—typically indicates upper gastrointestinal bleeding and represents a medical emergency. This occurs when blood is digested as it passes through the intestinal tract, resulting in the characteristic dark appearance. In contrast, stools may appear darker due to benign dietary factors (such as consuming iron supplements, bismuth-containing medications, liquorice, or foods like beetroot and blueberries) or simply due to dehydration and slower transit time.

Whilst there is no official link between Ozempic and dark stools, patients with diabetes may experience gastrointestinal complications, including gastroparesis, which could theoretically contribute to symptoms. Additionally, some individuals taking Ozempic may also be prescribed antiplatelet agents or anticoagulants for cardiovascular protection, which could increase bleeding risk. If you notice dark or black stools whilst taking Ozempic, it is essential to consider the full clinical context, including other medications, dietary intake, and accompanying symptoms. Any unexplained or persistent change in stool colour warrants prompt medical assessment to exclude serious underlying pathology.

When Dark Stools Require Medical Attention

Dark stools can occasionally signify serious gastrointestinal pathology, and recognising warning signs is crucial for patient safety. Melaena—characterised by black, sticky, tar-like stools with a distinctive offensive odour—is a hallmark of upper gastrointestinal bleeding. Common causes include peptic ulcers, gastritis, oesophageal varices, and Mallory-Weiss tears. This presentation requires urgent medical evaluation, as significant blood loss can lead to haemodynamic instability and anaemia.

You should seek immediate medical attention if dark stools are accompanied by any of the following symptoms:

• Dizziness, light-headedness, or fainting

• Rapid heartbeat or palpitations

• Shortness of breath or chest pain

• Vomiting blood or material resembling coffee grounds

• Severe or persistent abdominal pain

• Pallor, cold or clammy skin

• Confusion or altered consciousness

These features may indicate significant blood loss requiring emergency intervention. Call 999 if you experience severe symptoms such as collapse, severe chest pain, or heavy bleeding. Otherwise, contact NHS 111 or attend your local Emergency Department without delay.

Even in the absence of alarming features, persistent dark stools warrant investigation. Your GP may arrange blood tests to check for anaemia (full blood count), assess iron levels, and evaluate renal and hepatic function. Depending on clinical findings, further investigations such as upper gastrointestinal endoscopy (gastroscopy) may be indicated to visualise the oesophagus, stomach, and duodenum directly. NICE guidance (NG12) recommends urgent specialist referral for patients with unexplained iron-deficiency anaemia or suspected gastrointestinal bleeding. Early assessment ensures timely diagnosis and appropriate management, whether the cause is benign or requires specific treatment.

Other Gastrointestinal Side Effects of Ozempic

Gastrointestinal adverse effects represent the most frequently reported side effects associated with Ozempic therapy. These symptoms arise primarily from the drug's mechanism of action—specifically, the slowing of gastric emptying and modulation of gut motility. Understanding these effects helps patients and clinicians anticipate and manage treatment-related symptoms effectively.

Nausea is the most common gastrointestinal complaint, affecting approximately 15–20% of patients according to the MHRA SmPC. It typically occurs during treatment initiation or dose escalation and often resolves within several weeks as tolerance develops. Vomiting may accompany nausea in some individuals, though it is generally less frequent. Decreased appetite is also very common and may contribute to the weight reduction observed in some patients.

Diarrhoea affects around 8–10% of patients and may range from mild to moderately severe. Conversely, constipation occurs in approximately 5% of users, reflecting the variable impact of delayed gastric emptying on bowel function. Abdominal discomfort, bloating, and dyspepsia are also commonly reported.

Rarer but more serious gastrointestinal complications include acute pancreatitis, which has been reported in post-marketing surveillance. Patients should be counselled to seek immediate medical attention if they develop severe, persistent abdominal pain, particularly if radiating to the back, accompanied by nausea and vomiting. Gallbladder disorders, including cholelithiasis and cholecystitis, have also been associated with GLP-1 receptor agonists, which may be related to changes in bile composition.

Some patients may experience symptoms of indigestion or reflux while taking Ozempic. This may be related to the medication's effect on gastric emptying. If you experience persistent heartburn or regurgitation, discuss symptom management strategies with your healthcare provider, which may include lifestyle modifications or acid-suppressing medications.

Managing Digestive Symptoms While Taking Ozempic

Effective management of gastrointestinal side effects can significantly improve treatment adherence and quality of life for patients taking Ozempic. Most digestive symptoms are mild to moderate, transient, and can be successfully managed with practical strategies and, where necessary, symptomatic treatment.

Dietary modifications form the cornerstone of symptom management. Patients should be advised to:

• Eat smaller, more frequent meals rather than large portions

• Choose bland, easily digestible foods during the initial treatment period

• Avoid high-fat, fried, or heavily spiced foods that may exacerbate nausea

• Stay well hydrated, sipping water throughout the day

• Limit alcohol and caffeine intake

• Eat slowly and chew food thoroughly

Dose titration plays a crucial role in minimising side effects. According to the MHRA SmPC, Ozempic is initiated at 0.25 mg once weekly for four weeks, serving as a dose-escalation period rather than a therapeutic dose. The dose is then increased to 0.5 mg weekly, with further escalation to 1 mg and potentially 2 mg (after at least 4 weeks at each step) if additional glycaemic control is required. This gradual approach allows the gastrointestinal system to adapt progressively. If symptoms are particularly troublesome, discuss with your prescriber whether temporarily maintaining a lower dose might be appropriate. Never alter your dose without consulting your healthcare provider.

Timing of administration may also influence symptom severity. Some patients find that administering the injection on a day when they can rest if needed (such as a weekend) helps them manage initial side effects more comfortably.

For persistent nausea, your clinician may prescribe an anti-sickness medicine if appropriate. If constipation occurs, this can often be managed with increased dietary fibre, adequate hydration, and, if necessary, gentle laxatives such as macrogol.

When to contact your healthcare provider: If gastrointestinal symptoms are severe, persistent beyond the first few weeks, or significantly impact your daily activities and nutritional intake, contact your GP or diabetes specialist nurse. They can assess whether dose adjustment, additional supportive treatment, or alternative diabetes management strategies might be more appropriate for your individual circumstances.

If you suspect you are experiencing a side effect from Ozempic, you can report this through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or via the Yellow Card app). This helps the MHRA monitor the safety of medicines.

Frequently Asked Questions