Foul-smelling stool has a putrid and rotten odor. Usually, this smell is from foods that people eat. Sometimes though, foul-smelling stool can indicate health problem, such as an infection, an allergy, or colitis.

This article outlines some causes of foul-smelling stool, as well as information on diagnosis, treatment, and signs for seeking medical help.

There are many reasons stool can have a bad smell. These may include the foods people eat, food allergies or intolerances, infections, and any underlying medical conditions. This section lists a few of the possible reasons.

1. Viral or bacterial illnesses

Viral or bacterial infections and parasitic infections can affect digestion and cause foul-smelling stool or diarrhea. These include:

2. Antibiotics and infection

People taking antibiotics may get temporary stomach upset and smelly stool when antibiotics disrupt the delicate balance of good and bad bacteria within the gut.

Symptoms usually go away after finishing antibiotics, and the good gut bacteria return to usual.

Sometimes, antibiotics destroy so many good digestive bacteria that the harmful ones multiply out of control, causing infection.

3. Malabsorption syndromes

Malabsorption encompasses different conditions that can make it difficult to digest certain foods. This can often cause foul-smelling stool. Examples include:

Lactose intolerance

Lactose is a type of sugar in milk and other dairy products. The enzyme lactase breaks lactose into two smaller sugars. Then the gut can absorb the smaller sugars into the bloodstream.

A lactose-intolerant person does not produce enough lactase to break up the sugars. Loose, foul-smelling stool forms when the intestines draw water from the bloodstream to dilute the indigestible lactose sugar.

Milk allergy

A milk allergy is not the same as lactose intolerance. The American College of Allergy, Asthma & Immunology (ACCAI) says that people who have a milk allergy have an overactive immune reaction to milk products, similar to a peanut allergy reaction.

This immune reaction can include hives, rash, or lips, tongue, and throat swelling. In addition, bloody, strong-smelling stool can result because of immune system irritation of the gut.

Celiac disease

According to the American Gastroenterological Association (AGA), people with celiac disease experience an immune reaction to eating gluten, a protein in wheat, barley, and rye. Therefore, FALCPA also requires gluten ingredients to be listed on food packaging to assist celiac patients in avoiding this protein.

In celiac disease, the immune system overreacts to gluten and attacks the small intestine lining.

Continual attacks on the small intestine lining can cause difficulty absorbing vitamins and nutrients from food. This malabsorption problem can lead to fatty, unpleasant-smelling stool and other complications.

Ulcerative colitis

Ulcerative colitis is a chronic type of IBD. In ulcerative colitis, the colon’s lining becomes inflamed and develops ulcers.

Ulcerative colitis experts think this condition occurs when the immune system mistakes good intestinal bacteria for harmful bacteria.

In response, it sends an overabundance of white blood cells into the gut to help fight infection. Excess white blood cells continue to surge into the large intestine, causing chronic inflammation and bad-smelling stool with blood or mucus.

Crohn’s disease

Crohn’s disease is another type of IBD. It can cause persistent and bad-smelling diarrhea. It may affect any part of the gastrointestinal (GI) tract, from mouth to rectum, similarly to ulcerative colitis.

Chronic pancreatitis

Chronic pancreatitis is a persistent inflammation of the pancreas that worsens over time. As a result, a person may have an oily, fatty, foul-smelling, pale-colored stool.

Chronic pancreatitis causes permanent damage, which affects a person’s ability to digest food and make pancreatic hormones.

Exocrine pancreatic insufficiency

With exocrine pancreatic insufficiency (EPI), the pancreas does not produce enough digestion enzymes. This can lead to indigestion symptoms, which may include foul-smelling stool.

It can also lead to poor absorption of vitamins and minerals and unintended weight loss.

4. 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 pale, greasy, smelly stool and, eventually, malabsorption of nutrients, which can cause serious complications.

SBS can occur for many reasons. A common cause is the surgical removal of intestine parts after other unsuccessful treatments for inflammatory bowel disease (IBD).

Symptoms related to the foul-smelling stool can vary widely; some symptoms may be manageable from home, including:

Symptoms suggested to be checked out by a healthcare practitioner if they exceed 48 hours:

In most cases, a doctor can diagnose the reason for foul-smelling stool by conducting a physical examination and asking about a person’s medical history.

If answers are not clear with a physical exam, other diagnostic tests may include:

  • Stool test: This involves testing a person’s stool for blood, bacteria, viruses, or acidity related to lactose intolerance.
  • Blood test: This indicates whether a person is anemic, dehydrated, malnourished, or has an infection or reaction to an allergen.

Once a healthcare practitioner has performed initial testing, they may refer the person to a specialist for:

  • Hydrogen breath test: This test measures the amount of methane or hydrogen gas in your exhaled breath that can identify lactose intolerance or bad bacteria overgrowth.
  • Celiac disease blood tests: It is best to keep gluten in the diet before testing for celiac disease. Stopping gluten before testing can affect the outcome of the tests and interfere with diagnosis.
  • Genetic test: This involves analyzing a blood or saliva sample for a gene linked to lactose intolerance.
  • Skin-prick test: A healthcare practitioner will prick the skin with a needle and a milk product. Irritation at the site indicates an allergy.
  • Oral food challenge: A person consumes a tiny amount of an allergen with a doctor or allergist present.
  • Imaging techniques: Tests such as abdominal X-ray, ultrasound, computerized tomography scan, and magnetic resonance imaging can check for obstructions and loss of bowel function.
  • Biopsy of the GI tract: This involves a doctor removing a tiny piece of the gut or taking pictures during an endoscopy, sigmoidoscopy, or colonoscopy for analysis.

In most cases, symptoms will subside soon after finishing antibiotics or identifying food causes. In the meantime, the following home treatments may help reduce symptom severity:

  • drinking plenty of low-sugar, clear fluids such as water or broth
  • eating a bland diet of dry toast, rice, applesauce, and bananas
  • limiting wheat, dairy, high fiber foods, and diarrhea-causing foods that can further irritate the gut
  • using over-the-counter (OTC) lactase enzymes if a person suspects lactose intolerance
  • using OTC alpha-galactosidase enzymes when eating foods that cause gas and bloating, such as beans
  • decrease the amount of alcohol intake

Some OTC medications or remedies may cause more harm than good, so check with your pharmacist or doctor for recommendations on the following:

  • OTC anti-diarrheal medications: People can use these for diarrhea only if the stool is free of blood, mucous, or pus and the person is free of fever.
  • Probiotics: These restore good bacteria balance and decrease foul stool odors.
  • Stool deodorizers: These are available OTC after a healthcare professional approves their use.

The ACCAI also notes that a doctor or allergist may advise someone with a milk allergy to carry an epinephrine pen. These pens allow a person to self-inject epinephrine if anaphylaxis happens, a rare but potentially life threatening allergic reaction.

Doctors recommend that people with celiac disease follow a gluten-free diet, even after the gut has healed. The AGA notes that it is important for celiac patients to consult a dietician to make proper diet changes for intestine health.

A doctor will usually tailor SBS treatment to a person’s symptoms and the amount of bowel the condition affects. Some treatment options include anti-diarrheal medications and dietary adjustments.

Treatments aim to regulate the immune system and prevent inflammation flare-ups for ulcerative colitis and Crohn’s disease. Therapies may include taking anti-inflammatory medications and avoiding foods that trigger symptoms.

According to the National Pancreas Foundation, the treatments for chronic pancreatitis centers on relieving pain until pancreas swelling subsides. Adding pancreatic enzymes through pancreatic enzyme replacement therapy (PERT) can help reduce foul smelling stool.

In many cases, prevention of smelly stool depends on eliminating certain foods, managing diseases that lead to stool problems, and seeking treatment when symptoms do not resolve.

Keeping a journal of foods and symptoms will assist healthcare providers with diagnosis and treatment.

In addition, it is always a good idea to practice good hand hygiene and wash any fruit or vegetables before consuming them to limit the risk of infection.

It is advisable to see a doctor right away if a person’s stool smells unusually strong and is accompanied by any of the following symptoms:

  • rectal bleeding, pus, or mucus
  • black or pale stool
  • chills, night sweats, or fever
  • cramping, abdominal pain, or tenderness
  • pain in the upper abdomen and back, which worsens when eating or drinking
  • sudden bowel incontinence for no other reason
  • weight loss or gain, or muscle loss for no other reason
  • confusion, lethargy, or fatigue
  • hives or anaphylaxis

These symptoms may show a more serious underlying health issue that needs prompt medical attention. In particular, experiencing sudden incontinence or sudden loss of sensation in the perineal area can also be signs of a neurological emergency.

Smelly stool usually resolves itself by changing the foods you eat, drinking more water, and incorporating probiotics into your eating plan.

If unusual symptoms accompany the foul odor for greater than 48 hours, a person may want to check with their doctor. Quickly addressing problems will increase the likelihood of a full recovery or optimal condition management.

The following are answers to additional questions about foul stool.

Is foul-smelling poop normal?

Sometimes foul-smelling poop can be a reaction to a person’s food. However, you should see a doctor if the bad smell is accompanied by other symptoms such as unusual colored stool, blood or pus in the stool, or fever is not healthy and would be best checked by a doctor.

Learn more about the colors of poop.

Is foul-smelling poop a sign of cancer?

Foul-smelling stool that appears pale and greasy could be a sign of pancreatic insufficiency, which can occur with pancreatic cancer. It is important that a person with these symptoms get checked by a doctor so that the medical professional can thoroughly evaluate their history along with any necessary lab work.

Why does my poop smell like sulfur?

Foods that contain a lot of sulfates, such as eggs, cabbage, broccoli, and cauliflower, can make stool have a sulfur (rotten egg) smell. This is usually harmless, but if the symptom continues, check with your doctor. Some people with sulfur-smelling stool may have giardiasis (beaver fever).

There are many reasons stool can smell foul. The source of the problem may include the food eaten, intolerance or sensitivities to certain foods, medicines used, infections, and underlying medical conditions.

If a person suspects a milk allergy is causing the problem, stopping milk and products containing milk will help return the gut to healthy function.

Those whose symptoms are antibiotic-related will find that the symptoms stop once they have finished the course. Other causes, such as celiac disease, short-bowel syndrome, ulcerative colitis, Crohn’s disease, and pancreatitis, will require medical treatment.

It is important that anyone worried about how their stool smells visit their doctor for a diagnosis and treatment.

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