What is the preferred method of iron replacement, intravenous (IV) iron infusion or oral iron supplements, for a patient with pre-existing constipation?

IV Iron Infusion is Preferred for Patients with Pre-existing Constipation

For patients who already struggle with constipation, intravenous iron should be the first-line treatment rather than oral iron supplements, as oral iron frequently causes or worsens constipation and other gastrointestinal side effects that lead to poor adherence and treatment failure. 1

Why Oral Iron Worsens Constipation

• Oral iron causes constipation as a direct side effect because unabsorbed iron accumulates in the intestinal lumen, with up to 70% or more of patients experiencing gastrointestinal issues including constipation 1, 2
• Only a small fraction of oral iron gets absorbed, meaning most of the daily 200 mg elemental iron requirement remains in the bowel, directly causing constipation 1
• In rare but documented cases, iron pills can cause severe complications including ileus and bowel obstruction from constipation 3
• Gastrointestinal side effects from oral iron lead to poor adherence, making treatment ineffective even when prescribed 2

Direct Indications for IV Iron in Your Patient

The American Gastroenterological Association explicitly recommends switching to IV iron for patients who cannot tolerate oral iron despite dosing adjustments 4, and pre-existing constipation represents a clear intolerance concern.

Additional guideline-supported indications that apply here:

• Patients intolerant to oral iron should receive IV iron as the preferred route 5, 6
• The European consensus on inflammatory bowel disease states that IV iron is advisable for patients intolerant to oral iron supplementation 5
• The FDA label for ferric carboxymaltose (Injectafer) specifically indicates its use for patients with intolerance to oral iron 6

Practical IV Iron Administration

• Use high-dose formulations like ferric carboxymaltose or ferric derisomaltose at 500-1000 mg per infusion, allowing complete iron repletion in 1-2 infusions rather than multiple treatments 4
• For patients weighing ≥50 kg, administer 750 mg IV in two doses separated by at least 7 days for a total of 1,500 mg per course 6
• These newer formulations have excellent safety profiles with true anaphylaxis being very rare (approximately 1 in 200 patients may experience complement-mediated infusion reactions) 2, 7
• IV iron must be administered in medical facilities by healthcare providers trained to manage potential hypersensitivity reactions 1

Safety Considerations

• Monitor serum phosphate levels, particularly with ferric carboxymaltose, as hypophosphatemia is a recognized side effect 6, 1, 7
• Previous concerns about IV iron increasing infection or cardiovascular disease risk are unfounded 2
• Avoid extravasation as brown discoloration at the site may be long-lasting 6
• Iron sucrose and iron gluconate have demonstrated safety in primary care settings with minimal adverse effects 8

Why Not to Trial Oral Iron First

While some guidelines suggest trying oral iron first in mild cases 5, this recommendation does not apply when the patient already has constipation. The European consensus acknowledges that oral iron should only be used in patients with clinically inactive disease who have not been previously intolerant 5, and your patient's pre-existing constipation represents a predictable intolerance.

• Oral iron causes dose-dependent side effects, with constipation being among the most common 5
• The unabsorbed iron remaining in the bowel can generate reactive oxygen species that may cause mucosal harm 5
• Even optimized oral iron dosing (once daily or every-other-day) still carries significant GI side effect risk 4

Monitoring After Treatment

• Check hemoglobin response within 4 weeks of IV iron therapy 4
• Monitor for recurrent iron deficiency every 3 months for at least a year after correction 5
• Re-treat with IV iron when serum ferritin drops below 100 μg/L or hemoglobin falls below 12-13 g/dL (depending on gender) 5

The evidence strongly supports bypassing oral iron entirely in your patient with pre-existing constipation and proceeding directly to IV iron replacement, which is more effective, better tolerated, and avoids worsening their existing bowel symptoms.