A sudden, unusual change in the scent of your stool can be unsettling, especially when the odor is described as metallic, solvent-like, or overly acidic—a “chemical smell.” Stool odor is typically caused by volatile organic compounds (VOCs) created when gut bacteria break down undigested food components. While a certain degree of pungency is normal, a distinct chemical scent warrants attention. The type and amount of these VOCs change based on what enters your digestive system and how effectively your body processes it, pointing to temporary dietary causes or underlying digestive issues.
Causes Related to Diet and Supplements
Certain items consumed can directly lead to a chemical smell in stool because they contain compounds that are not fully metabolized and are then released as odorous VOCs. High-dose vitamin supplements are a common culprit, particularly those in the B-complex family. Thiamine, or Vitamin B1, is an organosulfur compound, and sulfur compounds are known for their potent odors, which some people describe as metallic or sulfurous.
Artificial sweeteners, specifically sugar alcohols like sorbitol and xylitol, can also cause a sharp, chemical-like odor. Since the small intestine struggles to fully absorb these large molecules, they pass into the large intestine where gut bacteria rapidly ferment them. This fermentation produces large amounts of gas and short-chain fatty acids that contribute to the unusual smell. Highly processed foods containing strong flavorings or preservatives may also deliver compounds the body cannot easily neutralize, leading to a temporary shift in the fecal odor profile.
Digestive System Processing Issues
When the body fails to properly digest macronutrients, the resulting undigested material becomes a feast for colonic bacteria, which then generate highly odorous volatile compounds. This mechanical failure of digestion is often a primary cause of persistent, foul-smelling chemical odors.
One significant issue is fat malabsorption, known as steatorrhea, where the body cannot absorb dietary fat due to a lack of digestive enzymes or bile. Undigested fat moves through the intestines and is ultimately excreted, resulting in stool that is greasy, pale, and often has a pungent, sour, or chemical smell. This malabsorption can be caused by conditions affecting the pancreas, which produces fat-digesting enzymes, or the liver and gallbladder, which produce and store bile.
Carbohydrate malabsorption, such as in cases of lactose or fructose intolerance, also results in a chemical-like odor, often described as sickly sweet or acidic. When the body lacks the necessary enzymes, like lactase, to break down specific sugars, these carbohydrates travel to the colon intact. The fermentation of these sugars by bacteria is rapid and intense, creating a large volume of gas and organic acids that give the stool its sharp, acidic scent.
Bacterial Imbalances and Infections
The gut microbiota is largely responsible for producing the characteristic odor of feces, and an imbalance in this community can generate chemically distinct smells. Pathogenic bacteria or an overgrowth of typical bacteria can produce unique volatile organic compounds (VOCs) that differ from those of a healthy gut.
Specific infections are notorious for causing particularly foul or chemical smells. For instance, an infection with Clostridioides difficile (C. diff) is frequently associated with an unusually strong, sometimes described as sickly sweet, odor. This distinctive smell is often linked to the bacteria’s production of specific compounds, such as 4-methylphenol (p-cresol), or changes in bile acid levels. Additionally, Small Intestinal Bacterial Overgrowth (SIBO) involves an excessive amount of bacteria in the small intestine, leading to fermentation that occurs too early in the digestive tract and creates an excess of odorous gases and fermentation products.
Identifying Red Flags and Seeking Medical Advice
While temporary changes in stool odor are usually benign and linked to diet, a persistent chemical smell accompanied by other symptoms should prompt a consultation with a healthcare provider. Several associated symptoms are considered red flags that may indicate a serious underlying health issue.
You should seek medical attention if the unusual odor is accompanied by:
- Persistent diarrhea
- Unexplained weight loss
- Severe or chronic abdominal pain
- Blood in the stool
Other concerning signs include fever or stools that are consistently greasy, pale, or float easily, which may point toward a severe malabsorption issue. A doctor may investigate the cause through various tests, including stool analysis to check for signs of infection, blood tests to assess nutrient deficiencies, or imaging to evaluate the digestive organs.