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Underlying causes of diarrhoea

9-minute read

Key facts

  • Diarrhoea is often short term and can clear up in 3 or 4 days without treatment.
  • Some causes of diarrhoea are infectious; others are not infectious.
  • Viral gastroenteritis is a common cause of short-term diarrhoea and can clear up without treatment.
  • Diarrhoea caused by long-term conditions may be continuous, or it can come and go, or it may alternate with constipation.
  • Diarrhoea can be a side effect of some medicines, including antibiotics.

What causes short-term diarrhoea?

Some causes of diarrhoea involve infection by a virus, bacteria or parasites, while other causes are not infectious. Short-term diarrhoea is often due to a viral infection, such as viral gastroenteritis, and commonly clears up without needing treatment other than home care.

Viral infections do not respond to antibiotics. However, antibiotics may sometimes be prescribed to target a specific infection caused by bacteria (such as shigella) or for people who have a weak immune system and cannot adequately fight the infection.

Not all diarrhoea is short term, and it can sometimes be a symptom of a chronic, or ongoing problem or health condition.

Infectious causes of diarrhoea


Viral infections that cause gastroenteritis, such as rotavirus or norovirus, commonly cause vomiting and nausea, along with diarrhoea. These infections are often passed from person to person. They come on suddenly, but usually clear up in a couple of days.

People who are immunosuppressed or who have weakened immune systems — such as those living with cancer, HIV/AIDS or who have had an organ transplant — are more likely to develop ongoing diarrhoea as a result of a bout of gastroenteritis or other infectious diarrhoea.


Bacterial infections that can cause diarrhoea include salmonella, E. coli, and campylobacter. These infections are mostly acquired due to food poisoning.

In addition to diarrhoea, these infections commonly cause fever and blood in the stool (poo). They can also cause a cramping pain in the rectum. Anyone with fever or blood in their stool as well as diarrhoea should seek medical advice.

Travellers’ diarrhoea is often due to a bacterial infection.

Some types of bacteria cause food poisoning because of the toxins (poisons) they produce. These include staphylococcus aureus and bacillus cereus and you can become ill if you eat food contaminated with the toxins. Re-heated rice can cause food poisoning if the bacillus cereus toxin is present.

Vomiting, nausea and abdominal pain may come on within hours, while diarrhoea may take longer to appear.


Waterborne parasites, such as the parasites that cause cryptosporidiosis and giardia, are found in streams and lakes. They can also contaminate swimming pools. People commonly become infected with the parasites by swallowing contaminated drinking water — for example, tank water that is not adequately filtered — or from swimming in contaminated lakes, dams and rivers.

Anyone who has had diarrhoea should avoid swimming in a public pool for 2 weeks after recovery so they don’t spread the infection.

Non-infectious causes of short-term diarrhoea

There are other causes of short-term diarrhoea that are not infectious, several of which are related to diet:

Change in diet

A sudden change in diet can result in diarrhoea.

Spicy foods, such as those containing chillies can give some people diarrhoea.

Introducing fibre to the diet too quickly may give some people diarrhoea or problems with flatulence (farting).

Rich foods that are heavy in fats or oils can cause digestive problems for some people, including diarrhoea.

Certain food ingredients

The artificial sweeteners sorbitol and mannitol can cause diarrhoea and bloating, and are found in many sugar-free products.

The natural sugar fructose can also cause diarrhoea in some people.

Drinking too much alcohol

Diarrhoea is a common side effect of drinking too much alcohol. This is because alcohol speeds up digestion and can cause your stools to be loose. Long-term drinkers, however, may suffer from the opposite effect and have constipation.


Stress and anxiety can cause episodes of diarrhoea. And for people who have irritable bowel syndrome, their symptoms may be made worse by stress.

Which medical treatments can cause diarrhoea?

Some medical treatments may cause diarrhoea as a side effect:


Antibiotics can cause diarrhoea in a small proportion of people who take them. This is because some antibiotics are not selective in the bacteria they target, meaning they kill some 'good' bacteria as well as the 'bad' bacteria. This can alter the balance of bacteria in the bowel, resulting in a temporary bout of diarrhoea. This should clear up when the course of antibiotics is finished.

Clostridium difficile infection

Clostridium difficile (usually known as C. diff) is a bacterium that lives in the gut of many young children and some adults without causing any problems. It exists as part of a collection of bacteria and other organisms that live in the bowel.

Sometimes, however, C. diff can multiply out of control and take over, including after a person has taken antibiotics to treat an infection. The antibiotics often destroy good bacteria as well as the ones causing the infection. Without the good bacteria to compete with, C. diff can multiply unrestricted.

Once established in the bowel, C. diff can produce toxins that attack the bowel lining and cause diarrhoea and cramping abdominal pain. This may be mild or very severe. The infection may come back, even after it has been treated.


Many medicines can cause diarrhoea as a side effect. As well as antibiotics, common examples include antacids that contain magnesium; NSAIDs (non-steroidal anti-inflammatory drugs); anti-arrhythmic medicines designed to treat abnormal heartbeats; and laxatives.

Check the Consumer Medicine Information inside the packet for the list of potential side effects if you think your medicine may be causing diarrhoea. Your pharmacist or doctor will also be able to advise you.


Radiotherapy involving the abdomen or pelvis can affect your bowel, causing diarrhoea, cramping and flatulence. It may take weeks to settle after treatment is finished.


Chemotherapy can also cause bowel problems, including diarrhoea.

Which long-term health conditions can cause diarrhoea?

Some long-term (chronic) health conditions can cause diarrhoea. In such cases, the diarrhoea may be ongoing, but it can also come and go in episodes, or it may alternate with bouts of constipation.

Long-term health conditions that can cause diarrhoea include:

Inflammatory bowel disease

Crohn’s disease and ulcerative colitis are collectively known as inflammatory bowel disease. These conditions both cause flare-ups of gut symptoms, including diarrhoea that is watery and sometimes contains blood. Between flare-ups, the symptoms die down.

Irritable bowel syndrome

Irritable bowel syndrome is a bowel disorder with no physical cause. The symptoms include recurrent abdominal pain, which is linked to passing a stool (poo), a change in how often you pass stools, and how firm your stool is.

Some people will mainly have diarrhoea symptoms; others will have more constipation. The symptoms may be made worse by stress and anxiety.


Diverticula are small pouches that bulge out through the wall of the gut. They become more common as you get older. If the pouches become inflamed, this is called diverticulitis. Diverticulitis causes lower abdominal pain — usually on the left side — and diarrhoea or constipation. People of Asian descent often experience the pain on the right-hand side of the abdomen.

Food intolerances

A food intolerance is when your body can’t digest certain foods. Lactose intolerance is one example, where your body can’t digest lactose, the main sugar found in dairy products. The symptoms usually include gas (farting), bloating, and diarrhoea, after eating foods that contain the substance to which your body has an intolerance.

Anyone may have temporary lactose intolerance after a bout of gastroenteritis, but see your doctor if it does not clear up after a couple of weeks.


Malabsorption disorders are conditions where your digestive system cannot absorb enough nutrients from your food. This can lead to symptoms such as bloating, diarrhoea and foul-smelling fatty stools. The exact symptoms depend on the particular condition.

Cystic fibrosis and pancreatitis are other examples of conditions that can cause malabsorption. The disorder can also develop after bowel surgery, where a section of bowel has been removed.

Encopresis / constipation with overflow

Surprisingly, ongoing constipation can also cause diarrhoea. This happens when impacted stool sticks in the bowel, partially blocking it. Diarrhoea then leaks out around the hard stool, without any warning.

This faecal incontinence mostly occurs in children and is known as encopresis or constipation with overflow.


Parasites can cause ongoing diarrhoea in a person who has returned from travelling overseas. Sometimes this is referred to as 'traveller's diarrhoea'.

Resources and support

If you need advice on what to do if you have diarrhoea, call healthdirect on 1800 022 222 (known as NURSE-ON-CALL in Victoria) to speak with a registered nurse, 24 hours, 7 days a week.

For more information and support, try these resources:

For information in languages other than English:

For Aboriginal and Torres Strait Islander resources:

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: June 2021

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