What is the most effective oral iron preparation for iron‑deficiency anemia in adults?

Best Oral Iron Preparation for Iron-Deficiency Anemia in Adults

Start with ferrous sulfate 200 mg (65 mg elemental iron) once daily, taken on an empty stomach—this is the most cost-effective first-line option with equivalent efficacy to other formulations. 1, 2

First-Line Treatment Algorithm

Standard Initial Therapy

• Ferrous sulfate 200 mg tablet once daily (provides 65 mg elemental iron) 2
• Take in the fasting state to maximize absorption 2
• Alternative ferrous salts (ferrous fumarate, ferrous gluconate) offer no proven superiority despite marketing claims 1
• Cost comparison strongly favors ferrous sulfate at approximately $0.30-$4.50 per month versus $47.60 for newer agents like ferric maltol 1

Key Dosing Principles

Take iron only once daily—hepcidin levels remain elevated for 48 hours after a single dose, blocking further absorption and increasing side effects without improving efficacy 1. Multiple daily doses are counterproductive.

The optimal dose is 50-100 mg elemental iron once daily 2. Higher doses do not improve absorption due to hepcidin-mediated feedback but substantially increase gastrointestinal side effects (constipation 12%, diarrhea 8%, nausea 11%) 1.

When Standard Therapy Fails

For Intolerance to Daily Dosing

If patients cannot tolerate daily ferrous sulfate:

1. Alternate-day dosing (same dose every other day)—fractional iron absorption actually increases, with significantly lower nausea rates 2
2. Ferric maltol 30 mg twice daily—better tolerated in patients with inflammatory bowel disease or previous intolerance, though more expensive and slower iron loading 2
3. Do NOT switch between traditional iron salts (ferrous sulfate to ferrous gluconate, etc.)—this practice is not evidence-based 2

Absorption Enhancers

• Vitamin C 80 mg taken with iron on an empty stomach may improve absorption, though evidence is mixed 1
• Avoid tea and coffee within 1 hour of iron intake—these are powerful absorption inhibitors 1

Monitoring and Response Criteria

Early Assessment (Critical)

Check hemoglobin at 2-4 weeks after starting therapy 2:

• Expected response: Hemoglobin increase of ≥10 g/L (1 g/dL) within 2 weeks 1
• Failure to respond (Hb rise <10 g/L at 2 weeks) predicts ultimate treatment failure with 90% sensitivity 2

Duration of Therapy

Continue oral iron for 2-3 months after hemoglobin normalizes to replenish iron stores 2. Ferritin should increase within 1 month in adherent patients 1.

When to Switch to Intravenous Iron

Parenteral iron is indicated when: 2

• Oral iron is not tolerated despite alternate-day dosing
• No hemoglobin response after 2-4 weeks of adherent therapy
• Impaired absorption (post-bariatric surgery, active inflammatory bowel disease)
• Iron loss exceeds oral absorption capacity
• Chronic kidney disease with ongoing losses

IV iron formulations requiring only 1-2 infusions (ferric carboxymaltose, ferric derisomaltose) are preferred over multiple-dose regimens 1.

Common Pitfalls to Avoid

1. Prescribing multiple daily doses—this increases side effects without improving absorption due to hepcidin elevation 1
2. Switching between ferrous salts for intolerance—no evidence supports this practice; switch to alternate-day dosing or ferric maltol instead 2
3. Using modified-release preparations—these are less suitable for prescribing as iron may be released beyond the duodenum where absorption is optimal 2
4. Failing to monitor early response—waiting months to assess efficacy delays appropriate escalation to IV iron 2
5. Stopping iron when hemoglobin normalizes—stores require an additional 2-3 months to replenish 2

Special Populations

For patients with inflammatory bowel disease and previous intolerance to iron salts, ferric maltol demonstrated 63-66% hemoglobin normalization at 12 weeks with placebo-comparable side effects 2. However, parenteral iron remains more effective in active IBD with ongoing inflammation 2.

Bottom line: Despite newer formulations, ferrous sulfate once daily remains the evidence-based first choice due to equivalent efficacy, lowest cost, and decades of safety data. Reserve expensive alternatives for documented intolerance or treatment failure.