Causes of diarrhoea

Diarrhoea is when you need to poo more often than you normally do in one day. It usually means having 2 or more loose, watery or runny poo's (stools) in a 24 hour period. 

Diarrhoea can be a side effect of cancer treatment or the cancer itself. Having a combination of treatments can sometimes make diarrhoea worse. 

Let your doctor or nurse know if you get diarrhoea. They can give you advice about what you can do. It is very important you drink plenty of fluids, otherwise, you can easily get dehydrated. 

Cancer treatments

The following cancer treatments can cause diarrhoea:

Chemotherapy

Chemotherapy uses anti cancer drugs to destroy cancer cells. The drugs circulate throughout the body in the bloodstream. This is called a systemic treatment.

Chemotherapy damages the cancer cells and healthy cells that are constantly growing and dividing. For example the cells that line your digestive system .

This damage to the lining of the digestive system causes diarrhoea. Diarrhoea is a common side effect. It usually comes on in the first few days after each treatment. Some drugs can cause severe diarrhoea.

Radiotherapy

Radiotherapy is a type of ionising radiation (high energy) that destroys the cancer cells in the treated area. It does this by damaging the DNA  of these cells. Radiation also affects normal cells. This can cause side effects in the treatment area.

Diarrhoea is a common side effect if you have radiotherapy to the pelvic  area or back passage (rectum). You might also have stomach cramps or a lot of wind (gas, flatulence).

During radiotherapy the diarrhoea might be quite severe. Let your doctor or nurse know if this happens. It may last for a few weeks after the treatment ends. For many people it then gradually gets better.

Some people have long term changes to their bowels. If you have ongoing diarrhoea, your doctor might need to refer you to a hospital that specialises in bowel problems after pelvic radiotherapy.

Targeted cancer drugs

Targeted cancer drugs are a group of drugs that work in a number of different ways to try to stop cancer cells growing and dividing. Some of these drugs can cause diarrhoea. 

How bad the diarrhoea is will depend on the particular drug and the dose that you are having. Let your doctor or nurse know if you have diarrhoea. They will be happy to give you advice or treatment to help manage your diarrhoea.

Immunotherapy

Immunotherapy uses our immune system to fight cancer. It works by helping the immune system recognise and attack cancer cells. Some types of immunotherapy drugs are also called targeted treatments or biological therapies. 

You might have immunotherapy on its own or with other cancer treatments. A side effect can be diarrhoea which can be severe. If you have diarrhoea speak with the team looking after you.

Surgery

You might have diarrhoea after surgery. This can include surgery to your:

• food pipe (oesophagus)
• stomach
• gallbladder
• bowel
• pancreas

It might only be a short term problem for a few weeks or months. But some surgery might cause long term changes. Your doctor should discuss this with you before your operation. 

Stem cell or bone marrow transplant

Having a transplant means destroying as many cancer cells as possible and replacing these with healthy stem cells . You might have a transplant using your own stem cells or stem cells from another person.

A stem cell or bone marrow transplant is a treatment for some types of cancer, including:

• leukaemia
• lymphoma
• myeloma  

A transplant allows you to have high doses of chemotherapy and other treatments such as radiotherapy. 

Diarrhoea is a side effect of this treatment. The possible causes of the diarrhoea could be from:

• chemotherapy and radiotherapy
• infection
• graft versus host disease (GvHD)

GvHD is a possible complication of a transplant from another person. A symptom of GvHD of the digestive system is diarrhoea. This can be green and watery, and may look bitty. It sometimes contains mucus and blood. You might have cramping and abdominal pain.

Tube feeding  

You may need drip or tube feeding if you need extra nourishment during your treatment. Tube feeding can sometimes cause diarrhoea, due to having too much food or being fed too often.

Tube feeding is often temporary, the reasons why you may have this include:

• having problems with swallowing due to a cancer in the head or neck area
• having holes (fistulas) or an abscess in the food pipe (oesophagus) or stomach
• not being able to eat or drink for 5 days or more after an operation or other types of treatment such as side effects to the gut from a stem cell transplant

The cancer itself

Some types of cancer are more likely to cause diarrhoea than others, for example, bowel cancer.

If you have an advanced cancer, the cancer may cause diarrhoea. Depending on the cause, your doctor or nurse may be able to prescribe treatment to help control it.

Infections

Cancer treatments can weaken your immune system, which means you could get infections or stomach bugs and viruses more easily. Infections and viruses can cause diarrhoea. 

Side effects of other medicines

Other medicines that are not cancer treatment can also cause diarrhoea. These include:

• some antibiotics

• drugs to treat constipation (laxatives)

• medicines containing magnesium, such as some antacid medicines

• some anti sickness drugs, such as metoclopramide (Maxalon)

• some painkillers

• some medicines containing iron

Let your doctor or nurse know if you think you have diarrhoea because of medicines you are taking. They will be happy to help with advice or treatment.

Side effects of herbal supplements

Some herbal medicines can cause diarrhoea. These include milk thistle and ginseng.

We have only listed a couple of herbal supplements that may cause diarrhoea. There are many more, so do ask your doctor’s advice before taking any herbal supplements.

Remember that herbal products aren't necessarily all safe to take. Although they are natural products and you can buy them over the counter at a health shop, some may interfere with your cancer treatment.