Iron Side Effects
Iron supplementation commonly causes gastrointestinal side effects with oral formulations, while intravenous iron carries risks of infusion reactions, hypophosphatemia, and rarely infection, though serious adverse events are uncommon with modern formulations.
Oral Iron Side Effects
Common Gastrointestinal Effects
Oral iron supplementation is associated with dose-dependent gastrointestinal side effects that represent the primary limitation of this route 1:
- Constipation: 12% of patients
- Diarrhea: 8% of patients
- Nausea: 11% of patients
- Additional symptoms include abdominal pain, vomiting, dyspepsia, heartburn, and epigastric discomfort 2, 1
These side effects are dose-dependent, with ferrous sulfate showing significantly increased GI side effects compared to placebo (OR 2.32) and intravenous iron (OR 3.05) 3. Notably, doses of 80 mg elemental iron as ferrous fumarate are associated with significantly higher rates of constipation and laxative use compared to lower doses 4.
Mechanism of GI Toxicity
Unabsorbed iron in the intestinal tract generates reactive oxygen species through Fenton and Haber-Weiss reactions, triggering oxidative stress, lipid peroxidation, and potential intestinal epithelial cell damage 5. This can worsen intestinal inflammation and alter gut microbiota composition 6, 5.
Rare but Serious GI Effects
In rare cases, oral iron can cause acute ischemic-type gastritis with iron deposition, even in younger patients without significant comorbidities 7. This typically resolves rapidly with cessation of supplementation.
Intravenous Iron Side Effects
Infusion Reactions
Mild reactions occur in approximately 1:200 administrations, while major reactions occur in approximately 1:200,000 1. Most reactions are complement activation-related pseudo-allergy (CARPA) rather than true IgE-mediated allergic reactions 1, 8.
Symptoms include 9:
- Dyspnea and wheezing
- Hypotension
- Nausea and vomiting
- Abdominal pain
- Arthralgia and myalgia
- Urticaria and flushing
Critical management point: Avoid diphenhydramine and vasopressors during mild reactions, as these can convert minor reactions into hemodynamically significant events by exacerbating hypotension, tachycardia, and shock 8. Instead, stop the infusion for 15 minutes, then restart at 50% rate if symptoms resolve.
Hypophosphatemia
Treatment-emergent hypophosphatemia is now widely recognized, particularly with ferric carboxymaltose (FCM), occurring in 47-75% of FCM-treated patients versus <10% with other formulations 8. The PHOSPHARE-IBD trial reported 51% incidence of hypophosphatemia (<2 mg/dL) with FCM at any time from baseline to Day 35 8.
Symptoms include:
- Fatigue (mimicking iron deficiency anemia)
- Proximal muscle weakness
- Bone pain
- Asthenia, myopathy, and rarely respiratory failure
FCM-associated hypophosphatemia can be severe and prolonged up to 6 months, though the true duration remains unknown 8.
Infection Risk
A systematic review and meta-analysis found increased infection risk specifically in inflammatory bowel disease patients receiving intravenous iron 10. However, no statistically significant increased risk was demonstrated in other patient populations, though point estimates favored increased infection rates in nearly all groups 10. The risk appears context-specific, depending on baseline iron status, exposure to potential pathogens, and type of preparation 10.
Formulation-Specific Risks
Low molecular weight iron dextran carries a boxed warning for anaphylaxis risk and requires test dosing 2. High molecular weight iron dextran (Dexferrum) was removed from the market due to high adverse reaction rates 2.
Newer formulations (iron sucrose, ferric carboxymaltose, ferumoxytol, ferric derisomaltose) have improved safety profiles with lower anaphylaxis risk and no test dose requirement 2, 9.
Delayed Reactions
Flu-like symptoms, arthralgias, myalgias, and fever can occur hours to days after infusion, lasting up to 24 hours and responding to NSAIDs 8. Symptoms persisting beyond a few days require evaluation for other pathologies, including hypophosphatemia 8.
Practical Considerations
Oral iron tolerance strategies 1:
- Take no more than once daily (hepcidin remains elevated 48 hours, blocking further absorption)
- Consider every-other-day dosing to improve tolerance
- Limit to ≤100 mg elemental iron daily in IBD patients 6
- Take on empty stomach for better absorption, or with food if poorly tolerated
Black stools are a benign but common effect that increases with dose, occurring in 8% with ferrous bisglycinate 25 mg versus 31% with ferrous sulfate 50 mg 4.