Does intravenous (IV) iron supplements cause melena in patients?

Does IV Iron Cause Melena-Appearing Stool?

No, intravenous (IV) iron does not cause melena or melanotic-appearing stool because it bypasses the gastrointestinal tract entirely and is delivered directly into the bloodstream.

Why IV Iron Does Not Cause Dark Stool

• IV iron formulations are administered directly into the venous system and are taken up by the reticuloendothelial system for incorporation into hemoglobin or storage as ferritin 1
• Unlike oral iron supplements, IV iron does not pass through the gastrointestinal tract, so there is no unabsorbed iron reaching the colon to darken the stool 2
• The mechanism by which oral iron causes black stool—unabsorbed iron passing into the colon—simply does not occur with IV administration 2

Contrast with Oral Iron Supplements

• Oral iron supplements commonly cause stool to turn dark or black as a benign side effect, resulting from the majority of unabsorbed iron reaching the colon 2
• Only a small percentage of oral iron is absorbed in the small intestine, with most passing unabsorbed into the colon where it causes the characteristic black discoloration 2
• This is why oral iron supplementation is associated with high rates of gastrointestinal adverse effects including nausea, flatulence, and diarrhea, problems that are avoided with IV iron products 1

Clinical Implications

• If a patient on IV iron develops melena or black tarry stools, this represents true gastrointestinal bleeding and requires urgent evaluation 3, 4
• Hemodialysis patients receiving IV iron are particularly prone to occult GI bleeding from uremic enteropathy, uremic platelet dysfunction, and anticoagulation, with fecal blood loss quantified at 6.27 mL/day (2.2 L/year) 1
• Do not dismiss melanotic stools in patients receiving IV iron as a benign medication effect—investigate for pathological bleeding sources 3, 4

Important Caveat

• Gastric siderosis from oral ferrous sulfate can cause melena through mucosal injury, but this is specifically related to oral iron deposition in gastric mucosa, not IV administration 5
• The case reports of melena in patients receiving IV iron supplementation describe actual GI bleeding from underlying pathology (such as vascular malformations), not a medication effect 3, 6