Ferrous Sulfate Dosage for Iron Deficiency Anemia
For adults with iron deficiency anemia, give ferrous sulfate 50-100 mg elemental iron once daily (one 324 mg ferrous sulfate tablet containing 65 mg elemental iron), as this provides equivalent efficacy to higher or more frequent dosing with significantly fewer gastrointestinal side effects and better compliance. 1, 2
Adults
Standard dosing: 50-100 mg elemental iron once daily is the optimal regimen 2. This translates to one 324 mg ferrous sulfate tablet (65 mg elemental iron) once daily 3.
Once daily dosing is superior to multiple daily doses because it maximizes absorption while minimizing side effects 1. The 2024 AGA guidelines explicitly state to "give oral iron once a day at most" 1.
Alternate-day dosing (every other day) is an acceptable alternative for patients who cannot tolerate daily dosing, providing similar iron absorption with better tolerance 1, 4. Give 120 mg elemental iron (two tablets) every other day if using this approach 4.
Add vitamin C to enhance absorption 1. This is a simple, evidence-based adjunct that improves iron uptake.
Avoid modified-release preparations - they are less suitable for prescribing as iron is released beyond the duodenum where absorption is optimal 2.
Pregnant Women
For pregnant women, 60-65 mg elemental iron once daily is as effective as higher doses and causes fewer side effects 5, 6.
One 324 mg ferrous sulfate tablet (65 mg elemental iron) daily is the appropriate dose 5.
Once daily dosing (65 mg) versus twice daily (130 mg) shows no difference in preventing anemia, but once daily has significantly fewer gastrointestinal side effects (40% vs 71%, p=0.002) and better compliance 5.
Intermittent dosing (once or twice weekly) can be considered if daily dosing is not tolerated, though compliance decreases and side effects increase proportionally with dose frequency 6.
Children
For children aged 6 months to 8 years, give 2-3 mg/kg/day of elemental iron once daily 7, 8.
Ferrous sulfate is superior to iron polysaccharide complex in children. At 3 mg/kg/day, ferrous sulfate produced a 1.0 g/dL greater hemoglobin increase (p<0.001) and higher complete resolution of IDA (29% vs 6%, p=0.04) compared to iron polysaccharide complex 8.
For children under 4 years, use liquid formulations (ferrous sulfate drops 25 mg/mL elemental iron) 2, 7.
Do not crush or chew tablets in older children who can swallow them 3.
The 2 mg/kg/day dose provides substantial therapeutic benefit with high tolerability (only 4.8% drug-related adverse events) 7.
Monitoring and Response
Check hemoglobin at 2 weeks: Failure to increase hemoglobin by at least 10 g/L after 2 weeks of daily oral iron predicts treatment failure (90% sensitivity, 79% specificity) 2. This is your decision point to switch to IV iron.
Continue treatment for 3 months minimum to replete iron stores, then reassess ferritin 2, 7.
When to Switch to IV Iron
Switch to intravenous iron if 1:
- Patient does not tolerate oral iron despite once-daily or alternate-day dosing
- Ferritin levels do not improve after 2 weeks trial
- Conditions where oral absorption is impaired (inflammatory bowel disease, post-bariatric surgery, celiac disease)
Common Pitfalls
Avoid twice or three times daily dosing - this is outdated practice. The FDA label suggesting 2-3 times daily 3 contradicts current evidence showing once-daily dosing is equally effective with better tolerance 1, 2.
Don't use modified-release formulations - iron is released past the duodenum where it cannot be absorbed 2.
Don't continue ineffective oral therapy - if no hemoglobin response by 2 weeks, switch to IV iron rather than persisting with oral 2.
Gastrointestinal side effects are dose-dependent - if patients report side effects, reduce frequency to alternate-day rather than stopping treatment 1, 6.