Glucosamine is a widely used dietary supplement for joint health, particularly amongst people with osteoarthritis. Whilst many take it hoping to ease joint discomfort, some users experience digestive side effects. Does glucosamine cause diarrhoea? Yes, it can—diarrhoea affects up to 1 in 10 people taking glucosamine supplements. Understanding why this happens, how common it is, and what you can do to minimise digestive upset is important for anyone considering or currently using glucosamine. This article examines the evidence, explores the mechanisms behind gastrointestinal side effects, and provides practical guidance on safe use.

What Is Glucosamine and How Does It Work?

Glucosamine is a naturally occurring compound found in cartilage, the tough tissue that cushions joints. As a dietary supplement, it is commonly used to support joint health, particularly in people with osteoarthritis. Glucosamine supplements are typically derived from shellfish shells or produced synthetically in laboratories. The two main forms available are glucosamine sulphate and glucosamine hydrochloride, with sulphate being the most widely studied.

The proposed mechanism of action involves glucosamine serving as a building block for glycosaminoglycans and proteoglycans, which are essential components of cartilage matrix. In theory, glucosamine may support cartilage repair and maintenance, though this mechanism is not definitively proven. Some research suggests it may have mild anti-inflammatory properties, but the clinical significance remains uncertain.

The clinical evidence for glucosamine is mixed. While some studies have shown modest benefits in reducing osteoarthritis symptoms, other high-quality trials have found no significant advantage over placebo. The National Institute for Health and Care Excellence (NICE) advises not to offer glucosamine for osteoarthritis management due to insufficient evidence of benefit.

In the UK, most glucosamine products are classified as food supplements rather than medicines, meaning they are not subject to the same rigorous regulatory standards as prescription medications. This can result in variability in product quality. The typical recommended dose is 1,500 mg daily, taken either as a single dose or divided throughout the day (e.g., 500 mg three times daily).

Important safety considerations include avoiding glucosamine if you have a shellfish allergy (unless using synthetic forms), caution in people with asthma, and avoiding use during pregnancy or breastfeeding. People taking warfarin should seek medical advice before using glucosamine due to potential interactions affecting blood clotting. Those with diabetes should monitor their blood glucose levels when starting glucosamine. Always discuss supplement use with your GP or pharmacist, particularly if you have existing health conditions or take other medications.

Can Glucosamine Cause Diarrhoea?

Yes, glucosamine can cause diarrhoea in some individuals. According to product information, diarrhoea is classified as a common side effect, affecting up to 1 in 10 users. The severity typically ranges from mild loose stools to more frequent bowel movements, with severe or persistent diarrhoea being rare.

The likelihood of experiencing diarrhoea may depend on several factors, including the dose taken, the formulation used (sulphate versus hydrochloride), and individual susceptibility. Higher doses may increase the risk of gastrointestinal side effects. Some patients report differences in tolerability between formulations, though robust comparative data are limited.

Other gastrointestinal side effects commonly reported alongside diarrhoea include:

• Nausea and indigestion

• Abdominal bloating and discomfort

• Constipation

• Flatulence

It is important to distinguish between mild, transient diarrhoea that may resolve with continued use or dose adjustment, and persistent or severe symptoms that warrant medical attention. If diarrhoea is accompanied by blood in stools, severe abdominal pain, fever, or signs of dehydration (excessive thirst, reduced urination, dizziness), contact your GP or NHS 111 promptly. These symptoms may indicate a more serious underlying condition unrelated to glucosamine.

People with pre-existing gastrointestinal conditions may wish to exercise particular caution when starting glucosamine supplementation and monitor their symptoms closely.

If you experience any side effects from glucosamine, you can report them through the MHRA Yellow Card Scheme, which helps monitor the safety of supplements and medicines.

Why Does Glucosamine Affect the Digestive System?

The exact mechanisms by which glucosamine causes gastrointestinal side effects, including diarrhoea, are not fully understood. Several hypotheses have been proposed, though these require further research to confirm.

One possibility involves osmotic effects, particularly with glucosamine sulphate. When taken orally, poorly absorbed substances can draw water into the intestinal lumen, potentially leading to looser stools. This mechanism is similar to how certain sugar alcohols can cause digestive upset.

Glucosamine might also affect gut motility and transit time in some individuals. Changes in how quickly food moves through the digestive system could reduce water absorption time, potentially resulting in diarrhoea.

Some researchers have suggested the potential for mild irritation of the gastrointestinal mucosa in susceptible individuals. This irritation could manifest as various digestive symptoms, including diarrhoea, nausea, or abdominal discomfort.

The formulation and additives in glucosamine supplements may also contribute to digestive side effects. Many commercial preparations contain additional ingredients such as binders, fillers, flavourings, or other joint-supporting compounds (e.g., chondroitin, MSM). These excipients could independently cause gastrointestinal disturbances in some users.

Individual differences in digestive function and sensitivity likely play a significant role in determining who experiences side effects. While these explanations are plausible, it's important to note that the precise cause of glucosamine-related diarrhoea may vary between individuals and requires further scientific investigation.

How to Reduce Digestive Side Effects When Taking Glucosamine

Several practical strategies can help minimise the risk of diarrhoea and other gastrointestinal side effects when taking glucosamine supplements.

Take glucosamine with food: Consuming glucosamine alongside meals, particularly those containing some fat, can slow absorption and reduce direct irritation of the stomach lining. This simple measure often significantly improves tolerability.

Start with a lower dose: Rather than beginning with the full recommended dose of 1,500 mg daily, consider starting with 500 mg once daily for several days, then gradually increasing to the target dose over 1–2 weeks. This allows the digestive system to adapt to the supplement.

Divide the daily dose: If taking 1,500 mg daily, splitting this into two or three smaller doses (e.g., 500 mg three times daily) may be better tolerated than a single large dose.

Try different formulations: Some individuals may find that switching between different glucosamine products helps, though this is based on individual experience rather than strong evidence.

Ensure adequate hydration: Drinking plenty of water throughout the day supports normal digestive function and may help counteract any osmotic effects of glucosamine.

Consider timing: Some individuals find that taking glucosamine at a particular time of day (morning versus evening) reduces side effects, though this varies between individuals.

Review other medications and supplements: Glucosamine may interact with certain medications. Particularly important is the interaction with warfarin, which can increase INR and bleeding risk—if you take warfarin, consult your doctor before using glucosamine. People with diabetes should monitor their blood glucose levels when starting glucosamine.

If digestive side effects persist for more than 7 days despite these measures, or if diarrhoea becomes severe, contact your GP or NHS 111. For managing joint symptoms, NICE guidance recommends focusing on exercise, weight management if needed, and considering topical NSAIDs. Oral NSAIDs with stomach protection may be appropriate for some people. NICE advises not to routinely offer paracetamol, opioids, or glucosamine for osteoarthritis.

Frequently Asked Questions