Understanding Diarrhea -- the Basics

A number of noninfectious medical conditions may cause diarrhea. These include:

Inability to digest certain foods, including lactose intolerance (difficulty digesting sugar found in dairy products); celiac disease (an immune reaction to consuming gluten, a protein found in wheat, barley, and rye); and pancreatic problems, such as those caused by cystic fibrosis, which interfere with production of important digestive substances.
Surgery to remove part of your small intestine. A shortened small intestine may be unable to absorb all the substances you eat. This is referred to as short-bowel syndrome.
Surgical removal of the gallbladder. An increase in bile in the colon may result in watery stools.
Certain diseases of the endocrine (hormonal) system, including overactive thyroid disease, diabetes, adrenal disease, and Zollinger-Ellison syndrome
Certain rare tumors (including carcinoid tumor and pheochromocytoma) that produce diarrhea-causing substances (hormones)
Inflammation in the intestinal tract, which can result in chronic diarrhea. If you have inflammatory bowel disease (such as ulcerative colitis, Crohn's disease, or microscopic colitis), you will have regular bouts of diarrhea during a flare-up of your disease.
Irritable bowel syndrome, which may cause alternating bouts of diarrhea and constipation
Ischemic bowel disease, which can be caused by blocked arteries. Symptoms might include abdominal pain with bloody diarrhea.

Many medications can cause diarrhea. Some of the most common include antacids containing magnesium, laxatives, digitalis, diuretics, a number of antibiotics, chemotherapy drugs, cholesterol-lowering agents, lithium, theophylline, thyroid hormone, and colchicine.

Radiation therapy for prostate cancer or cancers in the abdomen can damage the intestine and cause diarrhea.

Toxins such as insecticides, psychedelic mushrooms, and arsenic can cause diarrhea, and overuse of caffeine or alcohol may contribute to diarrhea.