What is the recommended oral and intravenous iron regimen, dosing, and monitoring for an adult with iron deficiency anemia?

Iron Deficiency Anemia: Oral and IV Iron Regimen, Dosing, and Monitoring

For oral iron, prescribe ferrous sulfate once daily (or every other day for better tolerance) with vitamin C on an empty stomach; for IV iron, use single-dose or two-dose formulations (ferric carboxymaltose, ferric derisomaltose, or ferumoxytol) when oral iron fails, is not tolerated, or absorption is impaired. 1

Oral Iron Therapy

First-Line Approach

• Use ferrous sulfate as the preferred formulation—it is the least expensive and no formulation has proven superiority over another 1
• Typical elemental iron content: 65 mg per tablet of ferrous sulfate 1

Dosing Schedule

• Administer once daily at most—more frequent dosing increases side effects without improving absorption 1
• Consider every-other-day dosing (60-120 mg elemental iron) for improved tolerance with similar or equal iron absorption 1, 2
  • Hepcidin remains elevated for 24-48 hours after oral iron, blocking further absorption
  • Alternate-day dosing allows hepcidin to subside, maximizing fractional absorption 2

Optimization Strategies

• Add vitamin C (80 mg ascorbic acid) to improve absorption by forming iron chelates and reducing ferric to ferrous iron 1, 2
• Take on an empty stomach in the morning—avoid afternoon/evening doses as circadian hepcidin increases are augmented by morning iron 2
• Avoid tea and coffee within 1 hour of iron intake as they powerfully inhibit absorption 1

Common Side Effects

Expect constipation (12%), diarrhea (8%), and nausea (11%) 1

Intravenous Iron Therapy

Indications

Switch to IV iron when: 1

• Patient does not tolerate oral iron
• Ferritin levels fail to improve after a trial of oral iron (expect ferritin increase within 1 month; hemoglobin should rise 1 g/dL within 2 weeks)
• Conditions impairing absorption exist (post-bariatric surgery, active IBD, celiac disease with poor adherence to gluten-free diet)
• Active inflammation with compromised absorption (especially IBD)
• Iron loss exceeds oral absorption capacity

Preferred Formulations

Use high-dose formulations requiring 1-2 infusions rather than multiple-dose regimens 1:

• Ferric carboxymaltose: 750-1000 mg per dose (two doses of 750 mg one week apart OR single 1000 mg dose)
• Ferric derisomaltose: 1000 mg single dose
• Ferumoxytol: 510-1020 mg (two 510 mg doses OR single 1020 mg dose)

Caution: Ferric carboxymaltose carries risk of hypophosphatemia (affects 50-74% of patients), potentially causing bone pain, osteomalacia, and fractures 3

Dosing Considerations

• Mean doses often fall short of estimated iron need (e.g., 1244 mg given vs. 1580 mg needed) 4
• Calculate total iron deficit and aim to replace it fully
• High-dose, low-frequency administration is preferred over low-dose, high-frequency regimens 5

Safety Profile of IV Iron

Infusion Reactions

• True anaphylaxis is exceedingly rare (<1:200,000 administrations) 1, 6
• Most reactions (approximately 1:200) are complement activation-related pseudo-allergy (CARPA), not true allergic reactions 1, 6
• All IV iron formulations have similar risk profiles 1

Management of Reactions

• Mild reactions: Stop infusion, restart after 15 minutes at slower rate 1
• Severe reactions: Consider corticosteroids 1
• Avoid diphenhydramine—its side effects can mimic worsening reactions 1

Monitoring

Oral Iron

• Check hemoglobin at 2 weeks (expect 1 g/dL increase)
• Check ferritin at 1 month (should show increase)
• If no improvement, switch to IV iron 1

IV Iron

• Assess efficacy at 4-12 weeks post-infusion 4
• Monitor for hypophosphatemia, especially with ferric carboxymaltose (can cause "6H syndrome": hyperphosphaturic hypophosphatemia, high FGF23, hypovitaminosis D, hypocalcemia, secondary hyperparathyroidism) 3
• Target: hemoglobin normalization or increase ≥20 g/L 4

Special Populations

Inflammatory Bowel Disease

• Determine if IDA is from inadequate intake/absorption vs. GI bleeding 1
• Use IV iron with active inflammation and compromised absorption 1
• Treat active inflammation to enhance iron absorption

Post-Bariatric Surgery

• Use IV iron for IDA without identifiable chronic GI blood loss, as duodenal absorption is disrupted 1

Portal Hypertensive Gastropathy

• Start with oral iron initially
• Switch to IV iron if ongoing bleeding and no response to oral therapy 1