Irritable bowel syndrome (IBS) is an intestinal disorder that causes abdominal pain or discomfort, cramping or bloating, and changes in bowel movements (diarrhea, constipation, or both)
IBS is known by various other names such as irritable colon, spastic colon, spastic bowel, and functional bowel disease.
Muscle contractions in the colon push the stool toward the rectum. These contractions are controlled by nerves, hormones and by electrical activity in the colon musculature and result in defecation or bowel movement.

Causes

When colonic nerves become "irritated" the muscle contractions may become too hard (causing abdominal pain), may stop (causing constipation and bloating), or may accelerate. Irritated colonic nerves also make the colon very sensitive to distention which may also cause pain. The problem may occur because of stress, anxiety, or depression, or may be brought on by eating.

Symptoms

The main symptoms of irritable bowel syndrome are abdominal pain or discomfort that occurs along with constipation or diarrhea
Others symptoms of IBS are:

• Increased stool when abdominal pain starts
• Abdominal Bloating
• Anxiety may be present
• Alternating constipation and diarrhea
• Depression may be present
• Feeling the need for a bowel movement after just having one

Treatment

Conservative treatment of mild IBS involves changing the person's diet and alleviating stress, a high fiber, low fat diet is recommended. Fat strongly stimulates contractions of the colon and fiber keeps the colon mildly distended, which may help prevent spasms. Foods such as whole grain breads and cereals, beans, fruits and vegetables are good sources of fiber. Some doctors may suggest using an over-the-counter fiber supplement. Use of fiber is most important for patients with predominantly constipation as their main complaint. Fiber may not be as useful for patients with predominantly bloating or diarrhea.
Avoiding alcohol and caffeine are also important in controlling diarrhea in IBS.
Get plenty of exercise. Exercise reduces stress, and can help you maintain regular bowel activity. Recommended lifestyle changes consist of regular exercise, alleviating stress, getting a good night's sleep, reserving enough time to have a bowel movement

Medication may be used along with lifestyle changes to manage symptoms of irritable bowel syndrome (IBS).
The goal of medication treatment is to relieve your symptoms enough to prevent them from interfering with your daily activities, because it may not be possible to eliminate your symptoms. Medications may be prescribed to treat moderate to severe pain, diarrhea, or constipation that does not respond to home treatment.
Antidepressant medications may improve IBS symptoms even in people who do not have depression, especially the medication class known as tricyclic antidepressants.
Tegaserod (Zelnorm), a medication that increases movement of the intestines, is now approved to treat IBS in women whose main symptom is constipation.
Alosetron, a medication that decreases abdominal sensitivity, has been shown to relieve symptoms in women with IBS who do not have constipation. The drug can be prescribed only by doctors enrolled in a special program and is intended for severe cases of diarrhea-predominant IBS in women who haven't responded to other treatments. Alosetron is not approved for use by men.