GLP-1 receptor agonists such as semaglutide, dulaglutide, and liraglutide are widely prescribed for type 2 diabetes and weight management in the UK. Whilst these medications commonly cause gastrointestinal side effects including nausea, constipation, and diarrhoea, patients sometimes notice changes in bowel movement colour during treatment. Understanding whether GLP-1 therapy directly affects stool colour, when changes warrant medical attention, and how dietary modifications influence bowel appearance helps patients distinguish between expected effects and potential complications. This article examines the relationship between GLP-1 medications and stool colour, providing evidence-based guidance on managing digestive symptoms and recognising warning signs requiring clinical assessment.

How GLP-1 Medications Affect Digestive Function

Glucagon-like peptide-1 (GLP-1) receptor agonists, including semaglutide (Ozempic, Wegovy), dulaglutide (Trulicity), and liraglutide (Victoza, Saxenda), are increasingly prescribed for type 2 diabetes management and weight reduction. These medications work by mimicking the action of naturally occurring GLP-1, a hormone released from the intestinal tract following food intake. The primary mechanism involves stimulating insulin secretion in a glucose-dependent manner, suppressing glucagon release, and slowing gastric emptying.

The effect on gastric emptying is particularly relevant to digestive function. GLP-1 receptor agonists delay the rate at which food moves from the stomach into the small intestine, which contributes to increased satiety and reduced appetite. This effect is most pronounced early in treatment and may attenuate over time with long-acting formulations. This altered gastric emptying can modify the entire digestive process, affecting how quickly food is broken down and absorbed throughout the gastrointestinal tract. Additionally, these medications influence gut motility more broadly, potentially altering intestinal transit time.

Common gastrointestinal effects reported with GLP-1 therapy include:

• Nausea (commonly reported, especially during dose escalation)

• Constipation or diarrhoea

• Abdominal discomfort or bloating

• Reduced appetite and early satiety

Regarding bowel movement colour specifically, changes in stool colour are not listed as adverse effects in UK SmPCs for GLP-1 receptor agonists. The medications do not contain dyes or compounds known to alter faecal colour. However, the indirect effects on digestive transit time, dietary intake changes, and bowel habit alterations may influence stool appearance. Patients experiencing significant weight loss often modify their diet substantially, which can affect stool characteristics including colour, consistency, and frequency.

It's important to note that GLP-1 receptor agonists have been associated with rare but serious digestive complications, including pancreatitis and gallbladder disease, which can affect stool appearance. Any persistent or concerning changes warrant clinical evaluation to exclude underlying causes.

When to Seek Medical Advice About Stool Changes

Whilst GLP-1 medications commonly cause digestive symptoms, certain stool changes require prompt medical assessment. Understanding which alterations are potentially serious helps patients distinguish between expected side effects and warning signs of complications.

Seek urgent medical attention (contact 999, NHS 111, or attend A&E) if you experience:

• Black, tarry stools (melaena) – may indicate upper gastrointestinal bleeding

• Bright red blood in or on stools, particularly if accompanied by dizziness, weakness, or rapid heartbeat

• Pale or clay-coloured stools with yellowing of skin or eyes (jaundice) – potential biliary obstruction or liver dysfunction

• Severe abdominal pain with stool changes, especially if persistent or worsening

Stop your GLP-1 medication and seek urgent medical assessment if you develop symptoms suggestive of pancreatitis (severe, persistent abdominal pain which may radiate to your back, with or without vomiting).

Contact your GP within 24–48 hours for:

• Persistent pale or grey stools without jaundice

• Unexplained changes in stool colour lasting more than a few days

• Greasy, foul-smelling stools that float (steatorrhoea) – suggesting fat malabsorption

• Stool changes accompanied by unexplained weight loss, fever, or night sweats

• Severe or persistent diarrhoea (more than seven days) or constipation unresponsive to usual measures

It is particularly important to report stool changes if you have a history of gallbladder disease, pancreatitis, or inflammatory bowel conditions. GLP-1 medications and significant weight loss can increase the risk of gallstones, which may present with right upper abdominal pain, fever, and sometimes jaundice. The MHRA has issued guidance regarding the risk of pancreatitis with GLP-1 receptor agonists, and whilst rare, this serious adverse effect requires immediate medical attention.

When consulting your healthcare provider, note the duration, frequency, and specific characteristics of stool changes, along with any accompanying symptoms such as pain, nausea, or appetite changes. This information assists in determining whether investigations are warranted.

Managing Digestive Side Effects on GLP-1 Treatment

Most gastrointestinal side effects associated with GLP-1 therapy are mild to moderate and tend to improve over time as the body adjusts to treatment. Following the dose-escalation schedule in the product SmPC is important to minimise these effects, and several practical strategies can help manage digestive symptoms effectively.

Dietary modifications form the cornerstone of symptom management:

• Eat smaller, more frequent meals rather than large portions

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

• Stay well-hydrated, particularly if experiencing diarrhoea

• Increase dietary fibre gradually if constipated, alongside adequate fluid intake

• Limit carbonated beverages and foods that cause bloating

• Eat slowly and chew food thoroughly to aid digestion

For nausea management, patients often find relief by:

• Avoiding strong food odours during meal preparation

• Consuming bland, easily digestible foods such as toast, rice, or bananas

• Using ginger-based products (ginger tea or biscuits), which some patients find helpful

• Ensuring adequate rest and avoiding lying down immediately after eating

Constipation can be addressed through:

• Increasing water intake to at least 6–8 glasses daily

• Regular physical activity, which stimulates bowel motility

• Gradually increasing dietary fibre from fruits, vegetables, and whole grains

• Considering over-the-counter laxatives after consulting your pharmacist or GP

If side effects are severe or significantly impact quality of life, discuss with your prescriber whether dose adjustment or temporary treatment interruption might be appropriate. Some patients benefit from a slower titration schedule than standard protocols. Contact your prescriber promptly if you experience severe vomiting or signs of dehydration.

Never discontinue or adjust GLP-1 medication without medical guidance, particularly if prescribed for diabetes management, as this may affect glycaemic control. Your healthcare team can provide individualised advice balancing therapeutic benefits against tolerability.

If you suspect you are experiencing side effects from your GLP-1 medication, you can report these through the MHRA Yellow Card scheme (www.mhra.gov.uk/yellowcard).

Other Factors That May Affect Bowel Movement Colour

Stool colour varies considerably based on numerous factors unrelated to GLP-1 therapy. Understanding these variables helps contextualise any changes observed during treatment and prevents unnecessary anxiety.

Dietary influences are the most common cause of stool colour variation:

• Green stools: leafy vegetables (spinach, kale), green food colouring

• Red or reddish stools: beetroot, tomatoes, red food dyes, cranberries

• Orange stools: foods high in beta-carotene (carrots, sweet potatoes), rifampicin (an antibiotic)

• Dark brown to black: iron supplements, bismuth subsalicylate, liquorice, blueberries

• Pale or light-coloured: primarily caused by reduced bile flow into the intestine

Medications and supplements frequently alter stool appearance:

• Iron supplements commonly cause dark green or black stools

• Rifampicin and rifabutin can produce orange-red discolouration

• Bismuth subsalicylate causes black stools

• Some vitamins and herbal supplements contain pigments that pass through the digestive system

Medical conditions that may affect stool colour include:

• Pale/clay-coloured: biliary obstruction, hepatitis, cirrhosis, pancreatic disorders

• Black/tarry: upper gastrointestinal bleeding (peptic ulcers, gastritis)

• Bright red: lower gastrointestinal bleeding (haemorrhoids, diverticular disease, colorectal pathology)

• Yellow/greasy: malabsorption syndromes, coeliac disease, chronic pancreatitis

• Green: rapid intestinal transit, certain infections

Patients commencing GLP-1 therapy often simultaneously modify their diet significantly, increasing vegetable intake and reducing processed foods as part of weight management efforts. These dietary changes alone may alter stool characteristics. Additionally, the rate of intestinal transit affects how much bile pigment (which gives stool its brown colour) is broken down—faster transit may result in greener stools, whilst slower transit typically produces darker brown stools.

If you notice stool colour changes whilst taking GLP-1 medication, consider recent dietary intake and any new medications or supplements before attributing the change to your GLP-1 therapy. Keeping a brief food and symptom diary can help identify patterns and provide useful information for your healthcare provider if concerns arise.

Remember that persistent pale/clay-coloured, black tarry, or blood-stained stools warrant prompt medical assessment regardless of the suspected cause.

Frequently Asked Questions