It's an inconvenient fact of life that stool smells unpleasant, whether formed or liquid. The odor derives from the gases emitted by the stool, which is influenced by the mixture of bacteria in the colon, recently eaten foods, medical conditions and other factors. Some of the many conditions that cause diarrhea are associated with an unusual, foul stool odor. Common culprits include certain intestinal infections and medical conditions that lead to undigested fat or blood in the stool. Fecal odor, however, is not an objective or reliable indicator of the cause of diarrhea. Medical evaluation is needed for an accurate diagnosis and treatment.
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Clostridium difficile-Induced Diarrhea
Clostridium difficile (C. difficile) is a type of bacteria that can cause diarrhea and other more severe intestinal problems if it gains a foothold in the colon. People with a C. difficile infection typically experience loose to watery diarrhea with a distinctive foul odor and abdominal cramps. C. difficile-induced diarrhea occurs most often in people who have recently been treated with antibiotics. The illness predominantly affects seniors, especially those who have recently been hospitalized or live in a residential healthcare facility.
Giardiasis is an infection of the small intestine caused by a tiny parasite called giardia. Although many infected people do not develop symptoms, others develop a diarrhea and other digestive system symptoms. The stool may be partially formed to watery and often becomes greasy over time with a foul odor due to impaired absorption of dietary fats as well as sugars. This disruption in absorption of dietary nutrients can lead to weight loss. Other symptoms may include abdominal cramps, bloating, increased intestinal gas and belching, and possibly nausea. Giardia is the most common parasitic intestinal infection in the US and usually occurs due to consumption of contaminated water or food, especially raw fruits or vegetables.
Impaired absorption of dietary fats, proteins and/or carbohydrates -- also known as malabsorption -- occurs with a variety of medical conditions. With each of these ailments, the presence of these undigested, unabsorbed major nutrients typically leads to persistent diarrhea, bloating and increased gas. The presence of unabsorbed fat, in particular, causes foul-smelling stool. The odor may vary, depending on the amount of fat in the daily diet. Some of the many possible causes of malabsorption leading to foul-smelling diarrhea include:
- Pancreatitis, or inflammation of the pancreas
- Impaired bile production or secretion
- Cirrhosis of the liver
- Celiac disease, also known as gluten-sensitive enteropathy
- Crohn disease, an inflammatory bowel disorder
- Cystic fibrosis, a genetic disorder that affects production of pancreatic digestive enzymes
- Surgical removal of the stomach, or part of the small intestine or pancreas
- Cancer of the pancreas
Digestive System Bleeding
Digestive system bleeding often causes foul-smelling stool, which might be formed, semisolid or diarrhea. With bleeding higher in the digestive tract, such as caused by bleeding ulcers, the stool tends to be black and tarry with a putrid odor caused by partial digestion of the blood. With lower intestinal bleeding, the stool may appear maroon to bright red with a more metallic odor.
Next Steps, Warnings and Precautions
See your doctor as soon as possible if you experience foul-smelling diarrhea that persists for more than 7 days. Call your healthcare provider sooner if you have a weakened immune system, such as occurs with HIV/AIDS or cancer treatment, or if you are older than age 65. Seek immediate medical evaluation and treatment if you experience any warning signs or symptoms along with foul-smelling diarrhea, including:
- Dizziness, weakness or fainting
- Fever or chills
- Bright red bleeding from your rectum that is more than a few drops
- Severe or worsening abdominal pain
- Inability to keep down fluids for more than 12 to 24 hours
Reviewed and revised by: Tina M. St. John, M.D.
- AACN Advanced Critical Care: Clostridium difficile Infection and Fecal Microbiota Transplant
- FASEB Journal: Volatile Organic Compounds From Feces and Their Potential for Diagnosis of Gastrointestinal Disease
- Merck Manual Professional Version: Giardiasis
- Differential Diagnosis of Common Complaints, 6th Edition; Robert H. Seller and Andrew B. Symons
- Merck Manual Professional Edition: Overview of Malabsorption
- Epocrates: Evaluation of Steatorrhea
- Merck Manual Professional Edition: Overview of GI Bleeding