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How to Treat Constipation Caused by Your Medications

Constipation is when you do not have a bowel movement (go poop) as often as usual. Cancer treatments and other medications may cause constipation.

If you have constipation that is not caused by medications, ask to speak with a BC Cancer dietitian or check out this page.


Constipation is when you do not have a bowel movement (“go poop”) as often as usual. Your stool (“poop”) is hard or painful to pass. You may have cramps, nausea (feeling queasy), or a feeling of fullness or mild pain in your abdomen (stomach area). 

Constipation can cause bleeding, painful hemorrhoids (piles), tears in the anus (the opening to your “bum”), or infection. 
Cancer treatments and some other medications may cause constipation.

Pain medications, called “opioids” (such as morphine, hydromorphone, oxycodone and Tylenol #3,) may cause constipation. Opioids slow down the movement of stool through your bowel (intestines). This gives your bowel more time to take the water out of your stool, making it hard, dry and difficult to pass.  

You cannot treat medication-related constipation by changing the type of food or liquid you eat and drink.  Eating more fibre will not help this type of constipation and might make it worse.

You cannot treat this type of constipation with walking or other types of exercise.  

Talk to your health care team if you become constipated. 

 You can treat this type of constipation with laxatives that you can buy at a drug store or supermarket. You do not need a prescription for these laxatives.

Laxatives work in two ways:
  • Stimulant laxatives help stool move along your bowel or
  • Osmotic laxatives help liquid stay in your bowel so stool does not become dry and hard.

Just as you need to eat every day, you must take laxatives every day for them to work.  


The first laxative you should try is the stimulant type called sennosides. It is mild and works for most people. You can buy sennosides in pills or in liquid. 


If you have a history of frequent bowel cramps (also known as “Irritable Bowel Syndrome”), you may want to try an osmotic laxative first, such as PEG

Steps to treat constipation that is caused by your medication:

Each person has a different bowel pattern. What is “normal” for you will be different than what is “normal” for another person. However, having a bowel movement should not be uncomfortable. You should not have to push too hard and your stool should be soft, but formed.

It is important to tailor your laxative doses based on your bowel pattern and goal. A treatment that is tailored for you is called a protocol. 

Look at the diagram below. 


Click here to enlarge diagram. 

Each step up the protocol has a higher dose so you can work up to the level you need to maintain a comfortable bowel pattern. If you move up the protocol as shown, you will not risk over-treatment, which could cause diarrhea.

You should start EITHER the sennosides protocol OR the PEG protocol. Do not start both at the same time

Most people find the sennosides protocol to be the easiest to take. Sennosides cost less than PEG but PEG works just as well. 

Whichever protocol you choose, start at step 1. Wait 24 hours. If you have a bowel movement within 24 hours that is soft and comfortable to pass, stay at step 1.

If you do not have a comfortable bowel movement within 24 hours, go to the next step in the protocol.

Only go to the next step if you are still feeling constipated after 24hrs. 

Keep moving up a step every 24 hours until you feel your bowel pattern is good for you.

If you have diarrhea, stop taking laxatives until you have a normal bowel movement. Restart the protocol at a lower step than you were on when the diarrhea happened. Do not take pills for diarrhea.

If you have very bad stomach cramps, stop taking sennosides and call your health care team.

Call your health care team if you:

  • Feel the protocol is not working for you. You may need an osmotic laxative call lactulose syrup. This is sometimes necessary when you have reached the highest step on your protocol and you are still constipated.
  • Are not sure how much laxative to take or when you should move to the next step of the protocol.
  • Are still constipated after getting to the top step of the protocol. You may need a combination of laxatives or a change in medication or pain management. 
  • Start having watery stools after a period of being constipated. There may be hard stool stuck in your rectum. If this happens, do not take pills for diarrhea.

DO NOT use liquid laxatives that have mineral oil or magnesium in them. Brands such as Magnolax®, Agarol®, or Milk of Magnesia® may not be safe to use. Talk to your health care team first.

DO NOT take bulk-forming laxatives like Metamucil® or Prodiem®. These are forms of fibre, which do not work for medication-related constipation. They will make bowel movements very hard if you do not drink enough fluids.

DO NOT take a suppository or use an enema before speaking with your health care team if you think your white blood cell or platelet counts might be low. Some cancer treatments increase your risk of having low white blood cells or low platelets. White blood cells help fight infection. Platelets help your body to stop bleeding.

DO NOT take Docusate (example: "Colace"). This is a stool softener. It does not help opioid-related constipation and we do not recommend that you take this


Revised June 2020

SOURCE: How to Treat Constipation Caused by Your Medications ( )
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