It's tricky to iron out differences among ORAL IRON supplements. Efficacy and tolerability are similar at equal doses of ELEMENTAL iron. But the percent of elemental iron varies between products.
For iron deficiency, start with 100 to 200 mg/day elemental iron in most cases. Use ferrous sulfate or fumarate, they often have 65 to 100 mg elemental iron/tab versus about 35 mg with ferrous gluconate. But more elemental iron can cause more GI upset and constipation. Advise patients to start with just one tab per day and titrate up to BID or TID over a week or two. Also explain that iron is best absorbed on an empty stomach, but it's okay to take it with food if GI upset occurs.
To limit GI upset, consider lowering the dose or giving it once daily, since some evidence suggests this may be enough for some patients. But it may take longer to correct an iron deficiency with a lower dose. Don't rely on stool softeners (Docusate, etc) to treat constipation. Vitamin C slightly increases iron absorption, 200 mg or 8 oz of orange juice increases absorption about 10%.
Be aware that slow-release or enteric-coated products may not be absorbed as well because they dissolve later in the GI tract. And there's no proven advantage to using polysaccharide-iron complex, carbonyl iron, or heme iron polypeptide. Monitor hemoglobin about once a month in iron-deficient patients.
REFERENCES
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Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015 May 7;372(19):1832-43. Available at: https://pubmed.ncbi.nlm.nih.gov/25946282
Dull RB, Davis E. Heme iron polypeptide for the management of anaemia of chronic kidney disease. J Clin Pharm Ther. 2015 Aug;40(4):386-90. Available at: https://pubmed.ncbi.nlm.nih.gov/25953602
Liu TC, Lin SF, Chang CS, Yang WC, Chen TP. Comparison of a combination ferrous fumarate product and a polysaccharide iron complex as oral treatments of iron deficiency anemia: a Taiwanese study. Int J Hematol. 2004 Dec;80(5):416-20. Available at: https://pubmed.ncbi.nlm.nih.gov/15646652