Causes of diarrhoea

All cancer treatments have side effects. Diarrhoea can be a side effect of

• Chemotherapy
• Radiotherapy
• Biological therapy
• Surgery
• Bone marrow or stem cell transplant

A combination of these treatments can cause more severe diarrhoea.

Chemotherapy

Some chemotherapy drugs irritate the lining of your digestive system so diarrhoea can be a common side effect of chemotherapy. It usually comes on in the first few days after each treatment.

With some chemotherapy drugs, diarrhoea can be quite severe. Your doctor will let you know if the drugs you are having will cause diarrhoea. If you have bad diarrhoea, remember you can easily become dehydrated. So try to drink plenty of water.

If you cannot drink enough, or you think you are losing more fluid in diarrhoea than you can replace by drinking, you must see your doctor.

If you do have diarrhoea after chemotherapy, tell your doctor or chemotherapy nurse. They can give you tablets to reduce the diarrhoea that you can take with your next chemotherapy treatment.

You may also wish to ask about soothing creams to apply around your anus. The skin is this area can become very sore and even broken with severe diarrhoea.

There is general information about the side effects of cancer drugs in the treatment section of CancerHelp UK.

Radiotherapy

Diarrhoea is a quite a common side effect of having radiotherapy to the abdominal or pelvic area. But radiotherapy affects people in different ways, so it's difficult to predict exactly how you will react. You may also have

• Stomach cramps
• A lot of wind (gas, flatulence)

Diarrhoea should disappear after a few days, but it can continue for some weeks after your treatment finishes. Apart from being unpleasant, diarrhoea can also make you feel weak and tired. So it’s important to drink plenty of fluids. Otherwise, you can easily become dehydrated.

Remember - contact the radiotherapy department or your doctor if your diarrhoea doesn't seem to be getting better. You may need ‘anti diarrhoeal’ drugs. Or you may just need advice about making changes to your diet or drinking more.

Biological therapy

Biological therapies are cancer treatments that use natural body substances to attack cancers. Some drugs such as interferon, cetuximab and erlotinib can cause diarrhoea.

The severity of your diarrhoea depends on the drug, and the dose, 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.

Surgery

Sometimes surgery to your stomach, surgery to your gall bladder and surgery to your bowel or rectum can cause diarrhoea. This may only be a short term problem. But sometimes it can continue for weeks or months after your surgery.

Your doctor should discuss this with you before your operation. It can be difficult, but there are ways of managing long term diarrhoea.

Bone marrow or stem cell transplant

If you have had a bone marrow transplant from a matched donor, you may develop a transplant reaction called graft-versus-host disease (GVHD). One of the symptoms of GVHD is diarrhoea.

Some types of cancers such as bowel cancer are more likely to cause diarrhoea than others. And some cancers can produce hormones and chemicals that cause diarrhoea. Doctors call this paraneoplastic syndrome.

If you have a very advanced cancer then diarrhoea caused by the cancer itself may be more of a problem. Depending on the cause, your doctor or nurse may be able to prescribe treatment to help control it.

Cancer treatments can weaken your immune system, which means you could get infections more easily. Some types of infections can cause diarrhoea.

Diarrhoea can be a side effect of many types of drugs. These include

• Some types of antibiotics
• Drugs given to treat constipation (laxatives)
• Medicines that contain magnesium, such as some types of antacid medicines
• Some anti sickness drugs such as metoclopramide (Maxalon)

If you think you are having diarrhoea because of drugs you are on, let your doctor or nurse know, and they will be happy to help with advice or treatment.

Some herbal medicines can cause diarrhoea. These include

• Milk thistle
• Ginseng
• Cayenne
• Saw palmetto

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

Remember, herbal products aren't necessarily all safe to take just because they are 'natural' and you can buy them over the counter at a health shop. Some may interfere with your cancer treatment. So it is very important to let your doctor know if you are taking any herbal medicine when you have cancer.

There is more information about herbal medicine in the complementary therapy section of CancerHelp UK. We are in the process of writing a more detailed section on the use of complementary therapies in cancer care. This will include more detailed information on the many types of herbal supplements, their uses and side effects.