What Causes Diarrhoea?

Diarrhoea is stool that is looser or more watery than normal, most often together with an increase in how frequently the bowels are opened. It is generally short-lived and settles on its own. Diarrhoea that is long-standing, that recurs often, or that is accompanied by other important symptoms should nevertheless have its cause investigated.

What are the commonest causes of diarrhoea?

Bowel infections

This is one of the commonest causes of acute diarrhoea. Viruses, bacteria and, more rarely, parasites can give rise to diarrhoea.

The diarrhoea may be accompanied by:

• Nausea and vomiting
• Abdominal pain and cramps
• Fever
• Weakness

Food poisoning

Diarrhoea of sudden onset, abdominal pain, nausea and vomiting can follow the consumption of food and drink contaminated with micro-organisms or with the toxins they produce.

Medicines

Antibiotics in particular can give rise to diarrhoea by altering the bowel flora.

Besides these, certain magnesium-containing products, metformin and various other medicines can also cause diarrhoea.

In severe and persistent diarrhoea developing during or after a course of antibiotics, assessment for Clostridioides difficile infection may be required.

Food intolerances

Some people cannot sufficiently digest or absorb particular foods.

In people with lactose malabsorption, for example, diarrhoea, wind, bloating and abdominal pain can appear after consuming milk and certain dairy products.

Irritable bowel syndrome (IBS)

In people whose diarrhoea is long-standing or recurs from time to time, one of the causes may be diarrhoea-predominant irritable bowel syndrome (IBS-D).

In these patients there is generally abdominal pain together with a change in bowel habit.

Inflammatory bowel diseases

Crohn’s disease and ulcerative colitis can cause long-standing or recurrent diarrhoea.

Where blood in the stool, abdominal pain, weight loss or anaemia accompany the diarrhoea in particular, these diseases may need to be investigated.

Coeliac disease

An immune reaction to gluten can cause damage to the small bowel and impair absorption.

Besides diarrhoea, there may be bloating, weight loss, anaemia or various nutritional deficiencies.

How long does diarrhoea have to last to matter?

By its duration, diarrhoea is generally assessed as:

Acute diarrhoea: shorter than 14 days,

Persistent diarrhoea: 14–30 days,

Chronic diarrhoea: longer than 4 weeks.

Diarrhoea lasting longer than 4 weeks in particular is chronic diarrhoea, and its cause needs to be investigated.

The most important problem in diarrhoea: loss of fluid

During diarrhoea it is not only water that is lost from the body, but electrolytes as well.

Fluid loss can be more significant in children, in older people and in those with accompanying illness in particular.

A dry mouth, excessive thirst, a reduction in the amount of urine, dizziness and marked weakness may be signs of fluid loss.

What can be done when there is diarrhoea?

In mild and short-lived diarrhoea the most important aim is to replace the fluid and electrolytes that have been lost.

• Take enough fluid.
• Suitable oral rehydration solutions can be used where required.
• Continue eating as far as this can be tolerated.
• Cut down very fatty and heavy foods temporarily.
• Pay attention to hygiene.

It is not right to use antibiotics for every case of diarrhoea. A significant proportion of acute diarrhoea does not require antibiotics.

Antibiotics should not be started without a doctor’s advice.

Can anti-diarrhoeal medicines be used?

They are not suitable in every patient.

In people with high fever, bloody diarrhoea or suspected serious bowel infection in particular, the indiscriminate use of medicines that reduce bowel movement can be harmful.

The choice of medicine should be made according to the patient’s condition and the cause of the diarrhoea.

When can diarrhoea be dangerous?

Medical assessment is required if any of the following is present:

• Blood in the stool
• Black, tar-coloured stools
• High fever
• Severe or steadily increasing abdominal pain
• Persistent vomiting
• Marked fluid loss
• A marked reduction in the amount of urine
• Unexplained weight loss
• Diarrhoea that wakes the person from sleep at night
• Diarrhoea that lasts a long time or recurs often
• Severe diarrhoea starting after a course of antibiotics
• Diarrhoea developing in people whose immune system is suppressed

How is long-standing diarrhoea investigated?

Colonoscopy is not needed for every case of diarrhoea.

The doctor first assesses how long the diarrhoea has lasted, the character of the stool, the medicines being taken, eating habits, any antibiotics used recently and the accompanying symptoms.

Where necessary:

• Blood tests
• Stool examinations
• Tests for infection
• Coeliac tests
• Colonoscopy and biopsy
• Gastroscopy
• Investigations for malabsorption

can be carried out.

Remember

Diarrhoea is not a disease; it is a symptom.

Most short-lived diarrhoea settles on its own. In diarrhoea that is bloody, severe, recurrent or long-standing, however, what matters is not merely stopping the diarrhoea but establishing its cause.