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Causes of diarrhoea

9-minute read

Key facts

  • Diarrhoea is often short term and can clear up after a few 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 is diarrhoea?

Diarrhoea is having 3 or more loose or liquid stools (poos) in one day. It's a symptom not a medical condition.

This article covers diarrhoea in adults and children over 12 years. See diarrhoea in children for information about the symptom in young children.

What causes acute (short-term) diarrhoea?

The most common cause of acute or sudden diarrhoea is infection of the digestive system. Gut infections can involve viruses, bacteria or parasites.

There are other causes of short-term diarrhoea that are not infectious. These include:

  • changes in your diet
  • drinking too much
  • stress
  • medical treatment

Infectious causes of diarrhoea


Viral infections that cause gastroenteritis include rotavirus or norovirus. Viral infections commonly cause vomiting and nausea, along with diarrhoea.

These infections are contagious (easily passed from person to person). They come on suddenly, but usually clear up in a couple of days.

Diarrhoea can be caused by viral gastroenteritis, which commonly clears up without needing special treatment after a few days. Viral infections do not respond to antibiotics.

If you are immunosuppressed or have a weakened immune system, you are more likely to get ongoing diarrhoea after gastroenteritis. People with weakened immune system are those who:


Bacterial infections that can cause diarrhoea include salmonella, E. coli, and campylobacter. These infections are mostly due to food poisoning. Many types of food are associated with food poisoning.

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

Vomiting, nausea and abdominal pain may come on within hours if you have food poisoning. Diarrhoea symptoms can take longer to appear.

Bacterial infections commonly cause fever and blood in the stool (poo) along with diarrhoea. They can also cause a cramping pain in your rectum. If you have a fever or blood in your stool as well as diarrhoea it's important to seek medical advice.

Antibiotics may be prescribed if a specific infection caused by bacteria (such as shigella) has been diagnosed.

Travellers' diarrhoea is often due to a bacterial infection.


Parasites, such as those which cause cryptosporidiosis and giardia, are found in streams and lakes. Waterborne parasites can also contaminate swimming pools.

You may become infected with parasites by drinking untreated water such as tank water that is not adequately filtered. You can also be exposed by swimming in contaminated lakes, dams and rivers.

You should avoid swimming in a public pool for 2 weeks after having diarrhoea. This is so that you 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 these are related to what you eat. In fact, your poo can say a lot about your health.

Changes in my diet

A sudden change in what you eat can result in diarrhoea.

This can happen if you start eating more:

  • fibre
  • fat
  • artificial sweeteners

Drinking too much alcohol

Diarrhoea is a common side effect of drinking too much alcohol. This is because alcohol speeds up your digestion and can cause your stools to be loose.

However long-term heavy drinking could also cause constipation.


Stress and anxiety can cause episodes of diarrhoea. If you have irritable bowel syndrome, your symptoms may be made worse by stress.

Can medical treatments cause diarrhoea?

Some medical treatments may cause diarrhoea as a side effect.


Taking antibiotics can sometimes lead to diarrhoea. This is because some antibiotics are not selective. This means they kill 'good' bacteria as well as the 'bad' bacteria.

An example of this is clostridium difficile (C. diff). C. diff is a bacterium that lives in the gut of many young children and some adults without causing any problems.

Sometimes after taking antibiotics, C. diff can multiply out of control. C. diff can produce toxins that attack your bowel lining causing diarrhoea and cramping abdominal pain. This may be mild or very severe. It can keep coming back, even after it has been treated.

Other medicines

Many medicines can cause diarrhoea as a side effect. Some common examples are:

  • metformin
  • colchicine — used to treat gout
  • weight loss medicines — both tablet and injections
  • laxatives

You can find out more about side effects in the consumer medicine information. This is found inside the packet or online.

If you think your medicine may be causing diarrhoea speak with your pharmacist or doctor.


Radiotherapy that involves your abdomen (tummy) or pelvis can affect your bowel. This can cause diarrhoea, cramping and flatulence (wind).

It may take weeks to settle after your treatment is finished.


Chemotherapy can also cause bowel problems, including diarrhoea.

What are the causes of chronic (long-term) diarrhoea?

Diarrhoea can sometimes be a symptom of a chronic (ongoing) health condition. In such people, the diarrhoea may be ongoing, but it can also come and go. Other people may have diarrhoea that alternates with bouts of constipation.

If you have diarrhoea for more than 14 days, see your doctor. Diarrhoea that continues to be a problem for 4 weeks or more is considered long-term.

Long-term health conditions that can cause diarrhoea are:

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.

If you have inflammatory bowel disease your stools may sometimes contain blood. Between flare-ups, your symptoms die down.

Irritable bowel syndrome

Irritable bowel syndrome is a bowel disorder with no clear physical cause. Your symptoms may be made worse by stress and anxiety and include:

  • abdominal pain when passing a stool (poo)
  • a change in how often you poo
  • changes in how firm your stool is

Some people will mainly have diarrhoea symptoms; others will have more constipation.

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.

Symptoms of food intolerance can be:

You may have temporary lactose intolerance after a bout of gastroenteritis. 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 a variety of symptoms such as:

  • bloating
  • diarrhoea
  • foul-smelling fatty stools

Cystic fibrosis and pancreatic diseases are examples of conditions that can cause malabsorption.

Malabsorption disorder can also develop after bowel surgery, when a section of your bowel has been removed.


Surprisingly, ongoing constipation can also cause diarrhoea. This happens when impacted stool sticks in your 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. This might be diagnosed if you have returned from travelling overseas.

Resources and support

For more information about chronic conditions associated with diarrhoea try these resources:

For health information about viral gastroenteritis in languages other than English visit NSW Health.

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.

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

Last reviewed: February 2024

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