The appearance of a chemical, metallic, or “motor oil” scent in stool suggests a change in the chemical composition of intestinal contents. Normal stool odor comes from volatile organic compounds created by gut bacteria as they break down food. A dramatic shift in this smell can be caused by simple dietary alterations or more complex digestive issues.
Temporary Causes from Diet and Medications
Changes in diet and certain medications are common reasons for a transient chemical odor in stool. Consuming highly processed foods or artificial ingredients introduces compounds that gut bacteria process into unfamiliar volatile organic compounds. High intake of supplements, such as large doses of B vitamins or fish oil, can also directly affect the resulting smell.
Prescription medications, especially antibiotics, frequently alter stool odor. Antibiotics disrupt the gut microbiome, reducing normal bacterial populations and allowing different chemical byproducts to form. This shift can result in a metallic or chemical scent that typically resolves after the medication course is finished. Iron supplements can also impart a metallic, rust-like smell to stool due to excess iron.
The Significance of Undigested Fat
The symptom most often associated with a greasy, rancid, or chemical smell is steatorrhea, which is the presence of excess fat in the feces. This occurs when the body fails to properly digest or absorb dietary fats, leaving them relatively unchanged. Undigested fat is chemically altered by colon bacteria, creating the oily or chemical odor.
Steatorrhea produces stools with distinct physical characteristics beyond the strong smell. They are typically pale, bulky, and difficult to flush because the high fat content causes them to float. An oily film or grease slick may also be left behind in the toilet bowl. Fat malabsorption can stem from issues like insufficient pancreatic digestive enzymes, bile duct blockages in the liver or gallbladder, or intestinal diseases like Celiac disease.
Other Digestive System Changes
A chemical or acrid smell can arise from changes in the bacterial environment or the speed of digestion, separate from fat malabsorption. Small Intestinal Bacterial Overgrowth (SIBO) involves an abnormal proliferation of bacteria in the small intestine that ferment food prematurely. This fermentation produces excess gases, including hydrogen sulfide, which can create a distinct sulfurous or chemical smell.
Gastrointestinal infections can also change stool odor by introducing pathogenic bacteria that produce unusual metabolic byproducts. Infections like Clostridioides difficile (C. diff) cause severe diarrhea with a distinctive, strong odor due to the compounds they release. Furthermore, extremely rapid transit time, such as during diarrhea, prevents normal decomposition, sometimes leading to a metallic or acrid scent.
When the Smell Requires Medical Evaluation
A single instance of unusual stool odor is often harmless. However, the smell requires medical evaluation if it is persistent, lasting more than a few days, or is accompanied by other warning signs.
You should consult a healthcare provider if the chemical smell occurs alongside concurrent symptoms. These symptoms may signal an underlying condition, such as a malabsorption disorder or a chronic infection, requiring specific diagnosis and treatment.
Red Flag Symptoms
- Unexplained weight loss
- Chronic abdominal pain
- A persistent change in bowel habits
- Fever
- Blood in the stool
- Visible presence of oil or grease in the toilet bowl