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From: Medicine.net
  
What causes constipation

Theoretically, constipation can be caused by the slow passage of digesting food through any part of the intestine. More than 95% of the time, however, the slowing occurs in the colon.

Medications: A frequently over-looked cause of constipation is medications. The most common offending medications include:

Narcotic pain medications such as codeine (e.g., Tylenol #3), oxycodone (e.g., Percocet), and hydromorphone (Dilaudid);
Antidepressants such as amitriptylene (Elavil) and imipramine (Tofranil) . Anticonvulsants such as phenytoin (Dilantin) and carbamazepine (Tegretol). Iron supplements
Calcium channel blocking drugs such as diltiazem (Cardizem) and nifedipine (Procardia).

Aluminum-containing antacids such as Amphojel and Basaljel
In addition to the medications listed above, there are many others that can cause constipation. Simple measures (e.g., increasing dietary fiber) for treating the constipation caused by medications often are effective, and discontinuing the medication is not necessary. If simple measures don't work, it may be possible to substitute a less constipating medication. For example, a non-steroidal anti-inflammatory drug (e.g., ibuprofen) may be
substituted for narcotic pain medications. Additionally, one of the newer and less constipating anti-depressant medications (e.g., fluoxetine or Prozac) may be substituted for amitriptylene and imipramine.

Habit: Bowel movements are under voluntary control. This means
that the normal urge people feel when they need to have a bowel movement can be suppressed. Although occasionally it is appropriate to suppress an urge to defecate (e.g., when a bathroom is not available), doing this too frequently can lead to a disappearance of urges and result in constipation.

Diet: Fiber is important in maintaining a soft, bulky stool. Diets that are low in fiber can, therefore, cause constipation. The best natural sources of fiber are fruits, vegetables, and whole grains.

Laxatives: One suspected cause of severe constipation is the over-use of stimulant laxatives (e.g., senna, castor oil, and certain herbs). An association has been shown between the chronic use of stimulanat laxatives and damage to the nerves and muscles of the colon, and it is believed that the damage is responsible for the constipation. It is not clear, however, whether the laxatives have caused the damage or whether the damage existed prior to the
use of laxatives and, indeed, has caused the laxatives to be used. Nevertheless, because of the possibility that stimulant laxatives can damage the colon, most experts recommend that stimulant laxatives be used as a last resort after non-stimulant treatments have failed.

Hormonal disorders: Hormones can affect bowel movements. For example, too little thyroid hormone (hypothyroidism) and too much parathyroid hormone (by raising the calcium levels in the blood) can cause constipation. At the time of a woman's menstrual periods, estrogen and progesterone levels are high and may cause constipation. However, this is rarely a prolonged problem.
High levels of estrogen and progesterone during pregnancy also can cause constipation.

Diseases that affect the colon: There are many diseases that can affect the function of the muscles and/or nerves of the colon. These include diabetes, scleroderma, intestinal pseudo-obstruction, Hirschsprung's disease, and Chagas disease. Cancer or narrowing (stricture) of the colon that blocks the colon likewise can cause a decrease in the flow of stool.

Central nervous system diseases: A few diseases of the brain and spinal cord may cause constipation, including Parkinson's disease, multiple sclerosis, and spinal cord injuries.

Colonic inertia: Colonic inertia is a condition in which the nerves and/or muscles of the colon do not work normally. As a result, the contents of the colon are not propelled through the colon normally. The cause of colonic inertia is unclear. In some cases, the muscles or nerves of the colon are diseased. Colonic inertia can also be the result of the chronic use of stimulant laxatives. In most cases, however, there is no clear cause.

Pelvic floor dysfunction: Pelvic floor dysfunction
(also known as outlet obstruction or outlet delay) refers to a condition in which the muscles of the lower pelvis that surround the rectum (the pelvic floor muscles) do not work normally.
These muscles are critical for defecation (bowel movement).
It is not known why these muscles fail to work properly in some people, but they can make the passage of stools difficult
even when everything else is normal.