Seeing blue stool can be unexpected, as most people expect stool to be brown or green. While unusual, it is generally harmless. This article explores common reasons for this color change and advises when to seek medical advice.
Common Dietary Causes
The most frequent reason for stool to appear blue is the consumption of foods or drinks containing strong blue, purple, or even some dark green pigments. These colorings, whether natural or artificial, can pass through the digestive system without being fully broken down or absorbed. As a result, the undigested pigment mixes with the stool, imparting a blue hue.
Common items include:
- Blue-colored candies, icings, frostings, certain cereals, and sports drinks.
- Naturally dark-pigmented fruits such as blueberries, blackberries, dark grapes, plums, currants, and raisins.
- Cooked red cabbage.
Normally, stool gets its typical brown color from bile, a yellow-green fluid produced by the liver that aids in fat digestion. As bile travels through the digestive tract, it undergoes chemical changes, gradually shifting from green to brown. However, when a significant amount of strong, indigestible blue dye is present, it can override this natural coloring process, resulting in visibly blue stool.
Other Potential Explanations
While diet is the primary cause, certain medications or supplements can also influence stool color. For instance, Prussian blue (Radiogardase), a medication used to remove radioactive compounds from the body, is known to cause blue feces. Methylene blue, sometimes used to treat a blood condition called methemoglobinemia, can also turn urine, sweat, and occasionally stool blue or blue-green.
Some iron supplements, though more commonly associated with green or black stool, might cause a bluish-green appearance. Medical conditions leading to blue stool are rare. One such condition is porphyria, which affects the body’s ability to break down heme, an iron-containing compound, and can result in blue or bluish-purple stool and urine, alongside other symptoms.
When to Consult a Doctor
If blue stool appears as an isolated occurrence, especially after consuming foods or drinks with blue or purple dyes, it is not a cause for concern. The color change is temporary and resolves once consumed pigments have passed through the digestive system, which can take a day or two.
However, if blue stool persists for several days without a clear dietary explanation, or if it is accompanied by other concerning symptoms, seek medical attention. These symptoms include persistent abdominal pain, fever, nausea, vomiting, diarrhea, unexplained weight loss, or blood in the stool. Consulting a healthcare professional is recommended if there is doubt or concern about changes in stool color or overall well-being.