This article outlines eight causes of foul-smelling stool, along with information on diagnosis, treatments, and when to see a doctor.
1. Antibiotics and infection
In some cases, antibiotics can cause foul-smelling poop.
Symptoms usually disappear soon after finishing a course of antibiotics, once the good gut bacteria have replenished.
Occasionally, antibiotics can destroy so many of the good intestinal bacteria that the harmful ones multiply out of control, causing infection.
People who experience an overgrowth of harmful intestinal bacteria while taking antibiotics may notice the following symptoms:
- watery, foul-smelling diarrhea, which may contain pus or blood
- pain, tenderness, and cramping in the abdomen
In most cases, a doctor can diagnose antibiotic-associated stomach upset by carrying out a physical examination and asking about a person's history of antibiotic treatment. They may also request a stool sample to check for bacterial toxins.
In most cases, symptoms will subside soon after a person finishes the course of antibiotics. In the meantime, the following home treatments may help reduce symptom severity:
- drinking plenty of fluids
- avoiding wheat, dairy, and high-fiber foods, which can further irritate the intestines
2. Lactose intolerance
Lactose is a type of sugar present in milk and other dairy products. The human body breaks lactose down, and an enzyme called lactase digests it.
A person who is lactose intolerant does not produce enough lactase to digest lactose.
People who are lactose intolerant may experience the following symptoms after consuming milk products:
- loose, foul-smelling stool
- bloating and gas
- abdominal cramping
People who suspect that they are lactose intolerant should eliminate all dairy products from the diet for several days. After the dairy-free period, a person should reintroduce milk or milk products to see whether symptoms return.
Other diagnostic tests include:
- Blood test: This indicates whether or not a person can successfully digest lactose after consuming products containing it.
- A hydrogen breath test: This involves a person repeatedly blowing into a bag after consuming lactose. If the collected air contains high levels of hydrogen, this indicates lactose intolerance.
- A stool acidity test: This involves testing a person's stool after they have ingested lactose. A highly acidic stool sample indicates lactose intolerance.
- A genetic test: This involves analyzing a blood or saliva sample for a gene linked to lactose intolerance.
- A surgical biopsy of the intestine: This involves a surgeon removing a tiny piece of the intestine for analysis.
The best way to prevent symptoms of lactose intolerance is to avoid milk and dairy products containing milk.
Or, people can buy tablets that contain the lactase enzyme. Taking the pills before eating dairy products can help the body digest lactose.
3. Milk allergy
The American College of Allergy, Asthma & Immunology (ACCAI) explain that having a milk allergy is not the same as having lactose intolerance.
People who have a milk allergy experience an immune reaction to milk and dairy products.
Symptoms of milk allergy include:
- bloody, foul-smelling stool
- upset stomach
- anaphylaxis, a rare but potentially life-threatening allergic reaction
According to the ACCAI, diagnostic tests include:
- A skin-prick test: A doctor dabs some milk onto the person's arm, then pricks the area lightly with a needle. Irritation at the site indicates an allergy.
- Blood test: This checks for immunoglobulin E antibodies, which the body produces in response to allergens.
- Oral food challenge: A person consumes a tiny amount of the allergen with a doctor or allergist present.
The only way to manage a milk allergy is to avoid milk and products containing milk.
The ACCAI also note that a doctor or allergist may advise a person who has a milk allergy to carry an epinephrine pen. These allow a person to self-inject epinephrine in the event of anaphylactic shock.
4. Celiac disease
A person with celiac disease should avoid products containing wheat, barley, or rye.
In celiac disease, the immune system overreacts to the presence of gluten and attacks the lining of the small intestine.
Ongoing damage can cause malabsorption, or an inability to absorb adequate nutrients from food. This can lead to further complications.
Common symptoms of celiac disease include:
- pale, fatty, or foul-smelling stool
- persistent bloating, gas, or abdominal pain
- persistent diarrhea or constipation
- weight loss or gain
- confusion, tiredness, and fatigue
- bone or joint pain
- tingling or numbness in the legs
- muscle cramps
- mouth sores
- a skin rash that itches
Standard diagnostic tests for celiac disease include blood tests and endoscopy. During endoscopy, a surgeon may remove a tiny piece of the small intestine to check for malabsorption.
A person should not eliminate gluten from the diet before undergoing these tests. Doing so can affect the outcome of the tests and interfere with diagnosis.
Following diagnosis, people with celiac disease should follow a gluten-free diet. The AGA note that the small intestine may take around 2 years to heal. Even after healing, people with celiac disease should continue to avoid eating gluten.
5. Short bowel syndrome
Short bowel syndrome (SBS) is a rare condition that occurs when a portion of the small or large intestine is absent or unable to function.
Because of this, people with SBS often experience malabsorption, which can cause serious complications.
SBS can occur for many reasons. A common cause is the surgical removal of part of the bowel following treatment for inflammatory bowel disease (IBD).
The symptoms of SBS vary among people but may include:
- pale, greasy, foul-smelling stool
- severe diarrhea
- weight and muscle loss
A doctor may request the following tests to help diagnose SBS:
- Blood tests: These can check for anemia, malnutrition, and dehydration.
- Imaging techniques such as abdominal X-rays and CT scans: These can check for obstructions and loss of bowel function.
- A liver biopsy: This can check liver function.
Doctors usually tailor SBS treatment to a person's symptoms and the amount of bowel the condition affects. Some treatments options include:
- anti-diarrheal medications
- medications to replace the intestinal lining
- dietary adjustments
- intravenous fluids
6. Ulcerative colitis
Ulcerative colitis is a chronic type of IBD. In ulcerative colitis, the lining of the colon becomes inflamed and develops ulcers.
Many experts believe that ulcerative colitis occurs when the immune system mistakes "friendly" intestinal bacteria for harmful bacteria.
In response, it sends white blood cells to the intestines to help fight infection. For some reason, this response fails to switch off, and white blood cells continue to flood the large intestine, causing chronic inflammation.
People with ulcerative colitis may develop a range of symptoms, including:
- foul-smelling stool containing blood or mucus
- bowel incontinence
- abdominal cramps
A doctor will conduct a physical exam and take a thorough medical history.
They may then opt for one or more of the following diagnostic procedures:
- blood tests
- stool analysis
- sigmoidoscopy or colonoscopy, which allow the doctor to see inside the large intestine with a camera
- endoscopy and biopsy
Treatment tends to focus on regulating the immune system and preventing inflammation flare-ups.
Treatments may include:
- taking anti-inflammatory medications
- avoiding foods that trigger symptoms
- eating a nutrient-rich diet
- undergoing surgery to remove the colon
7. Crohn's disease
Common symptoms include:
- an urgent need to empty the bowel
- persistent and foul-smelling diarrhea
- rectal bleeding
- a feeling of incomplete bowel-emptying
- abdominal cramps and pain
- fever and night sweats
- loss of appetite
- weight loss
Diagnostic tests for Crohn's disease include:
- blood and stool tests
- sigmoidoscopy or colonoscopy
- endoscopy and biopsy
Treatment for Crohn's disease tends to be similar to that for ulcerative colitis. However, doctors might target medications toward different areas of the GI tract.
People with severe Crohn's disease may undergo surgical bowel resection. This involves removing diseased sections of bowel and joining the healthy ends of the bowel together.
8. Chronic pancreatitis
Chronic pancreatitis can cause nausea and vomiting.
Chronic pancreatitis is persistent inflammation of the pancreas that worsens over time. Chronic pancreatitis causes irreparable damage, which affects a person's ability to digest food and make pancreatic hormones.
Symptoms may include:
- oily, fatty, foul-smelling stool
- pain in the upper abdomen and back, which worsens when eating or drinking
- pale or clay-colored stool
- malnutrition and weight loss
A doctor will carry out a physical exam and take a thorough medical history. They may also request the following diagnostic tests:
- CT scan
- magnetic resonance cholangiopancreatography, a type of MRI scan that uses dye to aid visibility of the internal organs
- abdominal ultrasound
- endoscopic ultrasound, in which a doctor inserts a flexible tube, or endoscope, into the small intestine via the mouth
According to the National Pancreas Foundation, the treatments for chronic pancreatitis tend to focus on relieving pain. They may include:
- taking pain medication
- undergoing Whipple procedure, or surgery to remove pancreatic lesions
- undergoing pancreatectomy, or surgery to remove all or part of the pancreas
When to see a doctor
People should see a doctor if their stool contains blood, is black or pale, or is accompanied by any of the following symptoms:
- abdominal pain
- weight loss
These symptoms may indicate a more serious underlying health issue that needs prompt medical attention.
Many factors can cause stool to smell foul. Such factors include the food that people eat, their allergies, the medication they take, infections, and any underlying medical conditions.
If a person suspects that a milk allergy is causing the problem, they should stop consuming milk and products that contain milk.
Those whose symptoms are antibiotic-related will find that the symptoms stop once they have finished the course. Other causes, such as IBD, Crohn's disease, and colitis, may require medical treatment.
Anyone worried about how their stool smells should visit their doctor for a diagnosis and treatment.