Elemental Iron Content in Ferrous Sulfate Formulations
Direct Answer
Both statements are correct—they refer to different formulations: ferrous sulfate 324-325 mg tablets contain 65 mg elemental iron, while ferrous sulfate 325 mg modified-release (MR) formulations contain 105 mg elemental iron. 1, 2
Standard Immediate-Release Formulation
The confusion arises from different pharmaceutical preparations:
- Ferrous sulfate 324-325 mg standard tablets contain 65 mg elemental iron 2
- This represents approximately 20% elemental iron content by weight 2
- The FDA drug label explicitly states: "Each tablet contains 324 mg of ferrous sulfate, equivalent to 65 mg of elemental iron" 2
- Multiple guidelines confirm this standard conversion: ferrous sulfate 200 mg contains 65 mg elemental iron, and ferrous sulfate 325 mg (standard formulation) contains 65 mg elemental iron 1, 3
Modified-Release Formulation
- Ferrous sulfate 325 mg modified-release (MR) tablets contain 105 mg elemental iron 1
- The British Society of Gastroenterology guidelines clearly distinguish between standard tablets (200 mg ferrous sulfate = 65 mg elemental iron) and MR formulations (325 mg ferrous sulfate MR = 105 mg elemental iron) 1
- MR capsules of 150 mg ferrous sulfate contain 48 mg elemental iron 1
Clinical Implications for Prescribing
When prescribing iron supplementation, you must specify both the salt form AND the specific formulation type (standard vs. modified-release), as elemental iron content varies significantly 1, 3:
- Standard ferrous sulfate 325 mg tablet = 65 mg elemental iron 1, 2
- Modified-release ferrous sulfate 325 mg tablet = 105 mg elemental iron 1
- Ferrous fumarate 325 mg = 108 mg elemental iron 1
- Ferrous gluconate 325 mg = 35 mg elemental iron 1
Recommended Therapeutic Dosing
The optimal starting dose for iron deficiency anemia is 50-100 mg elemental iron once daily 1, 3:
- A single standard ferrous sulfate 325 mg tablet (65 mg elemental iron) once daily is ideal for most patients 3, 4
- Doses of 60 mg elemental iron stimulate hepcidin levels, reducing subsequent iron absorption by 35-45% when taken multiple times daily 1, 3
- Higher doses do not improve absorption and increase gastrointestinal side effects 1
- For inflammatory bowel disease patients specifically, no more than 100 mg elemental iron per day is recommended 1