Why causes sour smelling poop?

Reviewed by Yoshinori Abe, MD

Internal Medicine

Sour-smelling poop happens when acids produced by gut bacteria change stool's normal odor. Common causes include dietary changes, lactose intolerance, malabsorption issues, or an imbalance of bacteria in your digestive system. In some cases, it may signal an underlying condition like a gastrointestinal infection, celiac disease, or inflammatory bowel disease.

Because sour-smelling stool can stem from something as simple as a new food or as serious as a chronic digestive disorder, it's worth investigating rather than guessing. A free, instant, online symptom check can help you quickly identify likely causes based on your specific symptoms and guide you toward the right next steps—whether that's a diet adjustment, a call to your doctor, or urgent care. It only takes a few minutes and could save you days of worry.

Reviewed for medical accuracy: 07/15/2026

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Explanation

Inside your digestive system, beneficial bacteria work together to break down the food you eat. During digestion, these bacteria produce various chemicals, including acids. When you change your diet or the balance of bacteria in your digestive system shifts, more acids may be produced—giving your stool a sour smell.

For example, eating lots of fruits, yogurt, or fermented foods can alter this acid balance. When there's a slight disruption in normal gut bacteria—known as dysbiosis—the increased production of acidic compounds can make your stool smell sour.

If the sour smell is persistent or associated with other symptoms like abdominal pain or changes in bowel habits, you can use this free symptom checker to help identify possible causes and determine whether you should consult with a healthcare provider about any underlying digestive issues.

(References)

Sekirov I, Russell SL, Antunes LC, Finlay BB. Gut microbiota in health and disease. Physiol Rev. 2010 Jul;90(3):859-904. doi: 10.1152/physrev.00045.2009. PMID: 20664075.
Chan DK, Leggett CL, Wang KK. Diagnosing gastrointestinal illnesses using fecal headspace volatile organic compounds. World J Gastroenterol. 2016 Jan 28;22(4):1639-49. doi: 10.3748/wjg.v22.i4.1639. PMID: 26819529; PMCID: PMC4721995.

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