When food leaves the stomach, it's still a partly digested mush. Your body recuperates valuable fluid from it while it's moved down the colon (large intestine). This transforms it into normal feces. The longer it stays in the colon, the drier it gets. This makes it harder.
Obviously, the quantity also increases if you wait to go to the toilet. A large, hard stool can be painful and difficult to pass. This can make people, especially children, reluctant to go, creating a vicious cycle. It's a common pattern of chronic constipation in children, which often begins when they start school. Many young children avoid school toilets and end up waiting too long.
Chronic constipation can last for months or years. It's usually caused by poor diet, some other disease, or regularly ignoring the urge to go to the toilet. Low-fibre diets and insufficient water intake are common causes of constipation.
While most otherwise healthy people will occasionally experience constipation, certain diseases or conditions can also cause it, such as:
- bowel obstructions due to a tumour or benign growth
- chronic kidney failure
- irritable bowel syndrome
- neurologic disorders such as Parkinson's disease, multiple sclerosis, or spinal cord injury
- rectal or colon cancer
Acute constipation starts suddenly and lasts for a few days. It can be caused by a blockage, prolonged inactivity, medication, dehydration, or missing a bowel movement. Pregnant women can develop constipation when the womb presses on the intestine. Sometimes, general anesthesia affects the bowel muscles for a few days after surgery. Lead poisoning and swallowing indigestible objects are other rare causes.
Examples of medications that can provoke acute constipation include:
- anticonvulsants used for epilepsy
- certain diuretics
- heart medications such as calcium-channel blockers
- iron supplements
- pain medications such as codeine* and morphine
- some cough and cold medications containing dextromethorphan
- some antacids
Overuse of laxatives eventually makes the bowels less sensitive to the need to eliminate feces and can cause chronic constipation. The bowels become dependent on laxatives to work, and this can lead to bowel distension and sometimes a condition called melanosis coli.
People who are bedridden can develop severe acute blockages called fecal impaction. The stools may have to be removed by their doctor.