Carbonyl Iron vs. Ferrous Sulfate

What's the Difference?

Carbonyl Iron and Ferrous Sulfate are both commonly used as iron supplements to treat iron deficiency anemia. However, they differ in their chemical composition and absorption rates. Carbonyl Iron is a pure form of iron that is highly bioavailable, meaning it is easily absorbed by the body. It is often preferred for individuals with sensitive stomachs as it is less likely to cause gastrointestinal side effects. On the other hand, Ferrous Sulfate contains iron in the form of ferrous ions and is less bioavailable. It may cause more digestive issues such as constipation or upset stomach. Therefore, the choice between Carbonyl Iron and Ferrous Sulfate depends on individual needs and tolerances.


AttributeCarbonyl IronFerrous Sulfate
Chemical FormulaFeFeSO4
AppearanceDark gray powderGreenish crystals
SolubilityInsoluble in waterSoluble in water
Iron ContentHigh iron contentLower iron content
UsesIron supplement, food fortificationIron supplement, water treatment
Side EffectsMinimal side effectsPossible gastrointestinal issues

Further Detail


When it comes to iron supplements, two commonly used forms are carbonyl iron and ferrous sulfate. Both of these compounds provide essential iron to the body, but they differ in terms of their chemical structure, absorption rate, side effects, and recommended usage. In this article, we will explore the attributes of carbonyl iron and ferrous sulfate, shedding light on their similarities and differences to help you make an informed decision about which supplement may be best for you.

Chemical Structure

Carbonyl iron, also known as iron pentacarbonyl, is an iron complex with the chemical formula Fe(CO)5. It is a bright yellow, crystalline solid that is highly stable and non-toxic. On the other hand, ferrous sulfate, also known as iron(II) sulfate, has the chemical formula FeSO4. It is a pale green or blue-green crystalline solid that is soluble in water. The difference in their chemical structures contributes to variations in their properties and how they interact with the body.

Absorption Rate

One of the key differences between carbonyl iron and ferrous sulfate lies in their absorption rates. Carbonyl iron is known for its slow and sustained release of iron, allowing for better absorption by the body over an extended period. This slow-release mechanism helps minimize gastrointestinal side effects, such as constipation and stomach upset, which are commonly associated with iron supplements. On the other hand, ferrous sulfate is rapidly absorbed by the body, but it may cause more pronounced side effects due to its higher reactivity and faster release of iron.

Side Effects

Speaking of side effects, both carbonyl iron and ferrous sulfate can cause certain adverse reactions, although the severity and frequency may vary. Carbonyl iron is generally well-tolerated, with minimal side effects reported. However, some individuals may experience mild gastrointestinal discomfort, such as nausea or diarrhea. In contrast, ferrous sulfate is more likely to cause gastrointestinal side effects, including constipation, stomach cramps, and dark-colored stools. These side effects can be mitigated by taking the supplement with food or using a slow-release formulation.

Recommended Usage

The recommended usage of carbonyl iron and ferrous sulfate also differs based on various factors, including the individual's iron deficiency severity, age, and overall health. Carbonyl iron is often recommended for individuals with mild to moderate iron deficiency, as it provides a slow and steady release of iron, reducing the risk of side effects. It is particularly suitable for individuals who cannot tolerate other iron supplements due to gastrointestinal issues. Ferrous sulfate, on the other hand, is commonly prescribed for individuals with more severe iron deficiency or anemia, as it offers a higher concentration of iron that is quickly absorbed by the body.


Both carbonyl iron and ferrous sulfate have been proven effective in treating iron deficiency and related conditions. However, the effectiveness may vary depending on the individual's specific needs and response to the supplement. Carbonyl iron's slow-release mechanism ensures a steady supply of iron over time, which may be beneficial for individuals with mild iron deficiency who require long-term supplementation. Ferrous sulfate, with its rapid absorption, is often preferred for individuals with more severe iron deficiency or those who need a quick boost in iron levels.

Interactions and Precautions

It is important to consider potential interactions and precautions associated with carbonyl iron and ferrous sulfate. Both supplements can interact with certain medications, such as antacids, proton pump inhibitors, and tetracycline antibiotics, reducing their effectiveness. Additionally, iron supplements should be used with caution in individuals with certain medical conditions, such as hemochromatosis or chronic liver disease. It is always advisable to consult with a healthcare professional before starting any iron supplementation regimen to ensure safety and efficacy.


In conclusion, carbonyl iron and ferrous sulfate are two commonly used iron supplements that provide essential iron to the body. While carbonyl iron offers a slow and sustained release of iron with minimal side effects, ferrous sulfate provides a higher concentration of iron that is rapidly absorbed. The choice between these supplements depends on factors such as the severity of iron deficiency, individual tolerance, and desired speed of iron absorption. Consulting with a healthcare professional is crucial in determining the most suitable iron supplement for your specific needs. Remember, maintaining optimal iron levels is essential for overall health and well-being.

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