Ferrous Sulfate Dosing for Iron-Deficiency Anemia
For iron-deficiency anemia, prescribe ferrous sulfate 200 mg (65 mg elemental iron) once daily in the morning on an empty stomach. 1, 2, 3
Standard Dosing Regimen
A single 200 mg ferrous sulfate tablet contains 65 mg of elemental iron, which falls within the optimal therapeutic range of 50–100 mg elemental iron per day. 1, 2, 3
The 324–325 mg ferrous sulfate formulation contains the same 65 mg elemental iron and is interchangeable with the 200 mg tablet. 4, 3
Take the dose in the morning, 1–2 hours before meals, to maximize absorption while the circadian hepcidin rise is lowest. 1, 5
Why Once-Daily Dosing Is Superior
Doses ≥60 mg elemental iron trigger a hepcidin surge that persists for 24 hours, blocking absorption of any subsequent dose by 35–45%. 1, 2, 5
Multiple daily doses increase gastrointestinal side effects (odds ratio 2.32 versus placebo) without improving total iron absorption due to this hepcidin-mediated blockade. 1, 6
Once-daily dosing produces similar hemoglobin responses to divided doses but with significantly fewer side effects. 1, 7
Alternate-Day Dosing for Intolerance
If daily dosing causes intolerable nausea, heartburn, abdominal pain, or constipation, switch to 100–200 mg elemental iron (two 200 mg ferrous sulfate tablets) every other day. 1, 5
Alternate-day dosing markedly increases fractional iron absorption because hepcidin levels subside by 48 hours, and it reduces gastrointestinal symptoms while preserving efficacy. 1, 5
Monitoring Response
Check hemoglobin at 2 weeks: an increase of ≥10 g/L predicts treatment success with 90% sensitivity and 79% specificity. 1, 8
Failure to achieve a ≥10 g/L rise at 2 weeks strongly predicts treatment failure and warrants investigation for non-adherence, ongoing blood loss, malabsorption, or concurrent vitamin B12/folate deficiency. 1, 8
Continue oral iron for 3 months after hemoglobin normalizes to fully replenish bone-marrow iron stores. 1, 8
When to Switch to Intravenous Iron
Escalate to IV iron if hemoglobin does not rise after 2–4 weeks of adherent oral therapy; early oral non-responders have only a 21% chance of later response versus 65% with IV iron. 1, 9, 10
IV iron is indicated when gastrointestinal side effects remain intolerable despite alternate-day dosing, or in malabsorption syndromes (inflammatory bowel disease, celiac disease, post-bariatric surgery). 1, 9
Modern IV formulations such as ferric carboxymaltose can deliver 500–1000 mg iron per infusion and produce a clinically meaningful hemoglobin increase within one week. 1, 9, 10
Common Pitfalls to Avoid
Do not prescribe 325 mg ferrous sulfate three times daily; this outdated regimen delivers 195 mg elemental iron per day, far exceeding the therapeutic target and causing unnecessary side effects. 4, 1
Do not switch between ferrous sulfate, ferrous fumarate, and ferrous gluconate expecting better tolerability; no evidence supports this practice. 1, 8
Do not use modified-release or enteric-coated iron preparations; they release iron beyond the duodenum and are classified as "less suitable for prescribing" by the British National Formulary. 1, 8
Do not rely on multivitamin preparations (typically ≤14 mg elemental iron) as sole therapy for iron-deficiency anemia. 1, 8