Intravenous Iron infusions causing dark or black stools doesn’t seem logical and yet some people seem to experience that. While I have seen this in my own patients and on online forums anecdotally, I couldn’t find any official studies specifically reviewing this. In my patients I have seen this more commonly with IV Ferrlecit (Iron Gluconate + Sucrose) and sometimes with IV Venofer (Iron sucrose). So let’s analyze this logically!
Reviewing the the pharmacokinetics of IV Iron preparations, we see that Lower molecular weight Iron preprations – such as Venofer and Ferrlecit – are more labile, and quickly release their iron that saturates Transferrin molecules in blood and then raises free iron i.e. NTBI (Non-Transferring Bound Iron) levels in blood which is known to induce oxidative damage to various organs including gastric/intestinal mucosa. Free Iron and also has been found in gastric / colonic mucosal interstitium which then perhaps freely or passively diffuses into the GI lumen . This 2019 case report discusses ‘gastric siderosis’ and ulceration induced by IV iron infusion. The more stable heavier molecular Iron preparations such as Iron dextran or Iron Carboxymol do not release their iron bounty in blood as easily, and have to be ingested via endocytosis by macrophages of the RES (Reticulocyte Endothelial system) to undergo digestion to release their Iron.
If dark stools happens in your patient after IV Iron infusions, this could either mean benign iron diffusion into the GI track discoloring stools or GI mucosal damage from free iron, or both !
Two things to consider doing is :
1. Slowing down the IV Iron infusion rate even more and
2. Add on a Proton Pump inhibitor or H2RA blockers such as Famotidine transiently to reduce some iron-gastritis and ulceration risk.
Similar risk exists with Oral Iron preps, especially in some vulnerable populations, and that would be yet another reason to avoid taking an oral Iron supplement more than every other day !
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