Medications that cause harmless black stools
Certain medications cause black stools not because of bleeding, but due to a non-harmful chemical reaction within the digestive system. This is often the case with over-the-counter and common supplements, and the stool color returns to normal after the medication is stopped.
Bismuth subsalicylate (e.g., Pepto-Bismol)
This medication is a very common cause of temporary, harmless black stools. When taken to treat upset stomach or diarrhea, the bismuth subsalicylate interacts with trace amounts of sulfur found in your saliva and gastrointestinal tract. The combination of these two substances forms bismuth sulfide, a black-colored salt that passes through your system, darkening the stool. This effect is temporary, and your stool color should return to normal within a few days of discontinuing the medication.
Iron supplements
Patients taking iron supplements for conditions like anemia often notice their stools turning a dark green or black color. This is because the body only absorbs a portion of the iron that is ingested. The unabsorbed iron travels through the digestive tract, where it reacts with the normal gut microbiome and other compounds, resulting in the dark stool. The effect is benign and not a sign of internal bleeding. Iron supplementation is a frequent cause of black stool during pregnancy, as many expectant mothers are prescribed iron to prevent or treat anemia.
Activated charcoal
Activated charcoal is a black, powder-like substance used in some health supplements and is also administered in emergency settings for certain types of poisoning. As it is not absorbed by the gut, it simply passes through the digestive system, turning the stool black as it goes.
Medications linked to gastrointestinal bleeding
In some cases, black or tarry stool, known as melena, is not a harmless side effect but a symptom of bleeding in the upper gastrointestinal tract. Certain medications can irritate the stomach lining or interfere with blood clotting, leading to this serious condition.
Nonsteroidal anti-inflammatory drugs (NSAIDs)
NSAIDs, such as ibuprofen, aspirin, and naproxen, can cause damage to the protective lining of the stomach and intestines. Long-term or high-dose use can lead to the formation of peptic ulcers, which may bleed. If a stomach ulcer bleeds, the blood is digested as it moves through the GI tract, causing the stool to appear black and tarry.
Anticoagulants and antiplatelet drugs
These medications are designed to reduce the risk of blood clots. Anticoagulants (like warfarin, apixaban, and rivaroxaban) and antiplatelet agents (like clopidogrel and aspirin) increase the risk of bleeding throughout the body, including the gastrointestinal tract. A patient taking one of these medications who develops black, tarry stools should seek immediate medical attention, as it could signal a severe bleeding event.
How to tell if black stool is from medication or bleeding
While alarming, black stool from medication or supplements is different from black, tarry stools caused by internal bleeding. Here is a quick guide to help distinguish between them:
- Appearance: Stool darkened by iron or bismuth is typically a solid, uniform dark brown or black. Bleeding-related stool (melena) is often described as jet black, sticky, and has a shiny, tarry appearance.
- Odor: Benign black stool has a normal smell. Melena, on the other hand, is characterized by a particularly foul and distinct odor, which is a byproduct of the digestion of blood.
- Other Symptoms: Benign black stool is generally an isolated symptom. Melena, especially if significant, may be accompanied by other signs of blood loss, such as fatigue, dizziness, weakness, or abdominal pain.
Comparison of black stools
| Feature | Harmless Black Stool (Medication/Supplement) | Melena (Internal Bleeding) |
|---|---|---|
| Appearance | Uniform dark brown or black; not typically sticky or tarry. | Jet black, tarry, sticky, and shiny. |
| Odor | Normal or typical stool odor. | Distinctly foul, unpleasant odor. |
| Associated Symptoms | Rarely accompanied by other symptoms. | Can be accompanied by dizziness, weakness, and abdominal pain. |
| Underlying Cause | Chemical reaction with bismuth, unabsorbed iron, or activated charcoal. | Bleeding peptic ulcer, gastritis, or irritation from NSAIDs and blood thinners. |
| Resolution | Resolves within a few days of stopping the medication. | Requires a medical diagnosis and treatment of the underlying cause. |
When to seek medical attention
If you have black stools, it is always wise to consult a healthcare provider for proper evaluation. Do not panic if you are taking a known cause like Pepto-Bismol or iron, but do not ignore the symptom, especially if you have not recently taken one of these medications. Medical attention is urgently required if your black stools are tarry, sticky, and foul-smelling, or if you experience accompanying symptoms such as:
- Abdominal pain
- Vomiting blood
- Dizziness or lightheadedness
- Sudden weakness
These can be signs of significant internal bleeding, and a healthcare provider will need to perform tests to determine the cause. Diagnostic procedures may include a stool test for hidden blood, blood tests for anemia, or an endoscopy to view the upper GI tract. For more detailed information on gastrointestinal bleeding, resources are available from institutions like the American Cancer Society.
Conclusion
Medications can be a common and harmless cause of black stools, most notably bismuth subsalicylate (Pepto-Bismol) and iron supplements. However, more dangerous culprits like NSAIDs and anticoagulants can cause bleeding that also results in black, tarry stool. Paying close attention to the characteristics of the stool, such as consistency and odor, as well as any other accompanying symptoms, is key to distinguishing a benign side effect from a medical emergency. If you have any doubt, it is always safest to consult a healthcare professional for a definitive diagnosis.
