What daily elemental iron dose should be used to start oral iron replacement in an adult with iron‑deficiency anemia?

Oral Iron Replacement Dosing for Iron-Deficiency Anemia

Start with 50–100 mg of elemental iron taken once daily in the morning on an empty stomach. 1

Recommended Formulation and Dose

• Ferrous sulfate is the gold-standard formulation and remains the most cost-effective option at approximately £1.00 for a 28-day supply. 1
• A standard ferrous sulfate 200 mg tablet provides 65 mg of elemental iron, which falls within the optimal 50–100 mg daily range. 1, 2
• Alternative ferrous salts can be substituted if ferrous sulfate is not tolerated:
  • Ferrous fumarate provides 69–106 mg elemental iron per tablet 1
  • Ferrous gluconate provides 37 mg elemental iron per tablet 1
• No single oral iron formulation has proven superiority in efficacy or tolerability over others. 1

Optimal Dosing Strategy

• Once-daily dosing is superior to multiple daily doses because iron doses ≥60 mg trigger hepcidin elevation that persists for 24 hours, reducing absorption of subsequent doses by 35–45%. 1, 3
• Morning administration is preferred because serum hepcidin increases during the day and reduces absorption of doses taken later. 1
• Take on an empty stomach (1–2 hours before or after meals) to maximize absorption, as food can reduce iron absorption by up to 50%. 1, 2
• Avoid tea, coffee, and calcium-containing foods for at least 1–2 hours after taking iron, as these inhibit absorption. 1, 4

Enhancing Absorption

• Co-administer with 250–500 mg vitamin C to enhance iron absorption by forming a chelate with iron. 1, 4
• This is especially important if you must take iron with food to improve tolerability. 1

Alternative Dosing for Side Effects

• If once-daily dosing causes intolerable gastrointestinal side effects, switch to alternate-day dosing (every other day) with 100–200 mg elemental iron. 1, 3
• Alternate-day dosing significantly increases fractional iron absorption compared to daily dosing because hepcidin levels subside by 48 hours. 1, 3
• This approach reduces side effects while maintaining efficacy, though the rate of hemoglobin rise may be slower initially. 1

Monitoring Response

• Check hemoglobin at 2 weeks: An increase of ≥10 g/L predicts treatment success with 90% sensitivity and 79% specificity. 1, 2
• Recheck hemoglobin at 4 weeks to confirm adequate response. 1, 2
• Continue treatment for approximately 3 months after hemoglobin normalizes to ensure adequate repletion of iron stores, not just correction of anemia. 1, 2

When to Consider Intravenous Iron

• Switch to IV iron if hemoglobin fails to rise ≥10 g/L after 2 weeks of adherent oral therapy, as this strongly predicts treatment failure. 1
• IV iron should be considered when:
  • Oral iron causes intolerable side effects despite dosing adjustments 1
  • Malabsorption conditions are present (inflammatory bowel disease, celiac disease, post-bariatric surgery) 1
  • Rapid correction is needed (severe anemia with Hb ≈7 g/dL) 4
• Modern IV formulations (ferric carboxymaltose, ferric derisomaltose) can replenish total body iron stores in 1–2 infusions of 500–1000 mg. 1, 5

Formulations to Avoid

• Do not prescribe modified-release preparations, as they release iron beyond the duodenum (the primary absorption site) and are classified as "less suitable for prescribing." 1
• Do not rely on multivitamin preparations, which typically contain insufficient elemental iron (≤14 mg) for treating iron-deficiency anemia. 1

Common Pitfalls

• Never prescribe multiple daily doses, as this increases side effects without improving absorption due to hepcidin-mediated blockade. 1, 3
• Do not discontinue therapy when hemoglobin normalizes—continue for 3 months to replenish iron stores. 1, 2
• Verify medication interactions, particularly with proton pump inhibitors and aluminum-based antacids, which impair iron absorption. 4, 2
• Investigate for ongoing blood loss if treatment fails, as continued bleeding will prevent adequate iron repletion. 4