The experience of stool odor is highly variable, but a smell described specifically as “burnt rubber” is unusual and often alarming. While the scent of feces is typically related to diet and the normal process of digestion, a strong, chemical, or rubber-like smell suggests an overproduction of specific volatile compounds in the gut. This distinct odor is usually a sign that metabolic or digestive processes are out of balance, typically involving sulfur-containing substances. Exploring the source can point to temporary dietary changes or, less commonly, underlying issues with how the body processes food.
The Chemistry of Unusual Stool Odors
The characteristic odor of human feces is a complex mixture resulting from the activity of the trillions of bacteria living in the colon. These microbes break down undigested food particles, which releases a variety of gaseous byproducts. The most potent and foul-smelling compounds are those that contain sulfur, which are readily detectable even in tiny concentrations.
The gases primarily responsible for the unpleasant smell are hydrogen sulfide, methanethiol, and dimethyl sulfide. Hydrogen sulfide is a major contributor, but the specific “burnt rubber” or chemical odor is likely caused by the unique combination or higher concentrations of complex methyl sulfides and thiols. These sulfur compounds are produced when gut bacteria metabolize sulfur-containing amino acids, such as cysteine and methionine, which are abundant in dietary protein.
Because the human nose has an extremely low detection threshold for these sulfur gases, a slight increase in their production results in a noticeable change in stool odor. The presence of excess sulfur-rich material in the large intestine provides the necessary substrate for sulfur-reducing bacteria. This ramps up the production of volatile compounds, resulting in a pungent odor sometimes described as chemical or burnt rubber.
Dietary and Medication Triggers
The most common cause of a chemical stool odor is a temporary change in diet or the introduction of certain medications or supplements. Foods naturally rich in sulfur provide an increased substrate for sulfur-reducing gut bacteria. If consumed in large quantities, the resulting increase in sulfur compounds can translate to a powerful, unusual stool smell.
Sulfur-Rich Foods
Sulfur-rich foods include:
- Red meat
- Eggs
- Cruciferous vegetables (like broccoli and cauliflower)
- Alliums (like garlic and onions)
Certain supplements and medications can also alter the gut environment to favor the production of sulfurous gases. Antibiotics, for example, disrupt the gut microbiota balance, potentially allowing sulfur-reducing bacteria to flourish. Iron supplements, particularly ferrous sulfate, contain sulfur and can lead to a noticeable sulfur smell as the compound is processed.
Supplements containing high levels of sulfur-rich compounds, such as certain B vitamins or glucosamine, also contribute to the odor. If these molecules are not fully absorbed in the small intestine, gut bacteria break them down in the colon, leading to an excess of volatile sulfur gases. A diet high in alcohol, which often contains sulfate, can also increase the availability of sulfate for colon bacteria to convert into sulfide gases.
Digestive Processing Issues
When the unusual odor is persistent and not easily explained by diet or medication, it often points to an issue with nutrient processing. Malabsorption is a primary driver, occurring when the small intestine fails to properly absorb fats, carbohydrates, or proteins. Undigested nutrients then pass into the large intestine, providing excessive food for colonic bacteria to ferment.
This excessive fermentation results in a higher volume of gas production, including the sulfurous compounds that create the “burnt rubber” smell. If the body cannot properly break down fat, the undigested fat molecules reach the colon and are fermented. This can lead to pale, greasy, and foul-smelling stool, a condition called steatorrhea. Similarly, conditions causing carbohydrate malabsorption, such as lactose intolerance, result in sugars reaching the colon where bacteria ferment them into various gases, altering the stool’s chemical profile.
An imbalance in the gut microbiota, known as dysbiosis, can also shift the type of gases produced. An overgrowth of specific sulfur-reducing bacteria, such as Desulfovibrio, can lead to a significant spike in hydrogen sulfide and other sulfur compounds. This microbial imbalance can occur due to long-term dietary habits, chronic stress, or underlying conditions like inflammatory bowel disease (IBD). The increased production of these sulfur gases requires the attention of a healthcare professional to identify the root cause.
Associated Symptoms and When to Seek Medical Advice
While an isolated, temporary change in stool odor is frequently harmless, the persistence of a chemical or burnt rubber smell, especially when accompanied by other symptoms, warrants medical evaluation. An unusual odor that lasts longer than a few days, or that cannot be linked to a specific dietary change, suggests a potential underlying digestive issue.
Symptoms Requiring Medical Attention
Accompanying symptoms that should prompt a consultation include:
- Severe or persistent abdominal pain
- Unexplained weight loss
- Chronic diarrhea or constipation
- Blood in the stool (bright red or dark and tarry)
- Fever, vomiting, or signs of dehydration
Digestive conditions like Celiac disease, Crohn’s disease, or chronic pancreatitis can lead to malabsorption, resulting in foul-smelling stools. Infections from bacteria like Clostridioides difficile (C. diff) or parasites like Giardia can also produce distinct, unpleasant odors and are often accompanied by severe diarrhea. When a change in smell is combined with systemic symptoms, it becomes an important diagnostic clue for a healthcare provider.