Diarrhoea is your digestive system hitting the emergency button. It is not a disease itself but a symptom that something has disrupted the normal flow of water and waste through your intestines. The real reason it happens is that your gut is either trying to flush out an irritant or failing to reabsorb fluid properly, resulting in loose, urgent stools.
Why Have I Got Diarrhoea? The Basic Mechanism
Your intestines process a huge amount of liquid every day. Food and drink enter, and digestive juices add more fluid. In a healthy gut, most of that water is absorbed back into your body as waste moves through the colon. The result is a formed stool.
Diarrhoea occurs when that balance breaks. Either the intestines secrete extra fluid, or they fail to absorb what they should. Sometimes the gut speeds up so much that water does not have time to be absorbed. In other cases, an irritant triggers the intestinal lining to release water in an attempt to dilute and flush out the problem.
That explains the volume and the urgency. Your body is actively trying to expel something it considers harmful, and it is using water to do it.
Infections Are the Most Common Cause
Most cases of acute diarrhoea come from infections. These are typically viral, bacterial, or parasitic.
Viral gastroenteritis is the leading culprit. Norovirus and rotavirus are well-known examples. These viruses infect the cells lining the small intestine, causing them to malfunction. The infected cells cannot absorb fluid properly, and they may actively secrete water into the gut. This type usually resolves on its own within a few days.
Bacterial infections come from contaminated food or water. Common sources include undercooked meat, unpasteurised dairy, and improperly handled produce. Certain bacteria release toxins that directly stimulate fluid secretion. Others damage the intestinal lining itself.
Parasitic infections are less common but worth considering if diarrhoea persists beyond a couple of weeks. These are often contracted through contaminated water sources, particularly when travelling.
Food Intolerances and Sensitivities
Some people develop diarrhoea because their bodies cannot break down certain food components. Lactose intolerance is a classic example. Lactose is the sugar in milk. If you lack enough of the enzyme lactase, undigested lactose travels to the colon where bacteria ferment it. That fermentation produces gas and draws water into the bowel, causing loose stools.
This is not an allergy. It is a digestive deficiency. Many adults naturally produce less lactase as they age, which is why dairy may start causing problems in your 30s or 40s even if it never did before.
Sugar alcohols like sorbitol and mannitol can cause similar issues. These are found in sugar-free gum and some processed foods. They are poorly absorbed and pull water into the intestines through osmosis.
Medications and Their Side Effects
Certain medications are well-documented causes of diarrhoea. Antibiotics are among the most common triggers. They do not discriminate between harmful bacteria and the beneficial bacteria that live in your gut. When antibiotics disrupt that balance, diarrhoea often follows.
Some antibiotics specifically allow a bacterium called Clostridioides difficile to overgrow. This can cause severe, persistent diarrhoea that requires medical treatment. If you have taken antibiotics recently and diarrhoea continues after you finish the course, that is worth discussing with a doctor.
Other medications that frequently cause loose stools include metformin for diabetes, certain blood pressure drugs, and medications that increase stomach acid production. Laxatives obviously cause diarrhoea if overused. Even antacids containing magnesium can have this effect in some people.
Chronic Conditions That Cause Diarrhoea
When diarrhoea lasts more than four weeks, it is considered chronic. At that point, infections become less likely as the cause, and underlying conditions deserve more attention.
Irritable bowel syndrome, or IBS, is one of the most common chronic causes. In IBS, the gut appears structurally normal, but the nervous system that controls it is hypersensitive. This can result in alternating constipation and diarrhoea, often triggered by stress or certain foods.
Inflammatory bowel disease is different. Conditions like Crohn’s disease and ulcerative colitis involve actual inflammation of the intestinal lining. This inflammation prevents normal fluid absorption and causes the gut to secrete fluid. Blood in the stool is more common with these conditions than with IBS.
Celiac disease is another possibility. In this autoimmune condition, gluten damages the small intestine’s absorptive surface. The damage reduces the gut’s ability to absorb water and nutrients, leading to diarrhoea and other digestive symptoms.
When to See a Doctor
Most cases of acute diarrhoea resolve within a few days without treatment. The primary risk is dehydration, so fluid intake matters most.
Medical attention is warranted in specific situations. Seek care if diarrhoea lasts more than two days without improvement, if you cannot keep fluids down, or if you notice blood or black, tarry stools. A fever above 102°F alongside diarrhoea also warrants evaluation.
For children and older adults, dehydration can develop faster. Signs include dry mouth, reduced urination, dizziness, and confusion. These groups should be monitored more closely.
Chronic diarrhoea lasting more than four weeks always deserves a medical evaluation. It is not something to manage indefinitely with over-the-counter remedies, as the underlying cause may require specific treatment.
Rehydration Is the Core of Treatment
For most cases, the treatment is straightforward: replace lost fluids and electrolytes. Water alone is often insufficient because diarrhoea depletes sodium, potassium, and other electrolytes.
Oral rehydration solutions are the gold standard. These contain a precise balance of salts and glucose that allows the intestine to absorb fluid efficiently. Commercial products are available at pharmacies. A simple homemade version can be made with clean water, salt, and sugar, but commercial preparations are more reliable in their composition.
Over-the-counter anti-diarrhoeal medications like loperamide can reduce stool frequency. They work by slowing gut movement. However, they should not be used if you have a high fever or blood in your stool, as these signs may indicate a bacterial infection that needs to be flushed out rather than slowed down.
Frequently Asked Questions
How long does diarrhoea usually last?
Acute diarrhoea typically resolves within 2 to 3 days. If it persists beyond 4 weeks, it is considered chronic and requires medical evaluation.
Can stress really cause diarrhoea?
Yes. The gut is directly connected to the brain through the nervous system, and stress can accelerate bowel movement. This is particularly pronounced in people with irritable bowel syndrome.
Should I take anti-diarrhoea medication or let it run its course?
For mild cases without fever or blood, anti-diarrhoea medication is generally safe. If you have a high fever or bloody stools, avoid these medications and seek medical advice instead.
Is it safe to eat dairy when I have diarrhoea?
Temporary lactose intolerance is common during and after a bout of diarrhoea. Even people who normally handle dairy well may find it worsens symptoms, so it is reasonable to avoid it until stools return to normal.
