Converting Ferrous Sulfate To Ferrous Gluconate

In our latest question and answer, the pharmacist discusses how to convert ferrous sulfate to ferrous gluconate.


Hello! I have SLE amongst other autoimmune issues. I have been taking ferrous sulfate 325mg twice daily, and by accident bought the ferrous gluconate. I opened the bottle so can’t return it now. Can I take this? What would the conversion be with this supplement? Everything I have read says it is fine, but I think it was saying you need 2 of the gluconate to equal the 325mg of the Ferrous Sulfate? Thank you in advance!!

Asked by Eileen On Feb 07, 2019

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Feb 07, 2019


Ferrous sulfate and ferrous gluconate are the two most common forms of iron used to both treat and prevent iron deficiency.

While the choice of either is generally based on personal preference, it is important to know that they both supply different amounts of elemental iron, so they are not directly interchangeable.

Ferrous sulfate contains far more elemental iron than ferrous gluconate, and sometimes, depending on the strength of ferrous gluconate you are using, you may need to take two to three tablets to get the same amount of iron as one ferrous sulfate tablet.

Elemental iron content, versus the content of the entire compound (e.g. ferrous sulfate; FeSO4), is a common point of confusion. Ferrous sulfate is the name of a compound, which contains both iron and a sulfate group:

Ferrous Sulfate Molecular Structure

Ferrous sulfate products provide 20% elemental iron by weight. This is why you may see a product label state two different strengths:

  • The amount for the whole compound (ferrous sulfate)
  • The amount of elemental iron

Here is an example product that lists 325 mg of ferrous sulfate, which provides 65 mg of elemental iron (20% of 325 equals 65):

Nature Made Iron

Ferrous gluconate contains less elemental iron than ferrous sulfate and is 12% elemental iron by weight. It actually is closer to 11.6%, but 12% is often referenced since it is a nice round number.

Like ferrous sulfate products, ferrous gluconate products also commonly list both the total compound strength and elemental iron strength. Here is a product containing 240 mg of ferrous gluconate, which provides 27 mg of elemental iron.

Ferrous Gluconate 240 Label Side

Converting Ferrous Sulfate And Ferrous Gluconate

While we can convert between ferrous sulfate and ferrous gluconate products in terms of equating their elemental iron, it is important to note that conversions aren't exact to the milligram. Nevertheless, you can convert between the two to obtain the same amount of elemental iron and be within a few milligrams.

Feel free to use this handy tool to do so.

Ferrous Sulfate - Ferrous Gluconate Conversion Tool

Ferrous Sulfate 

Elemental Iron 

Ferrous Gluconate 

*Note: The tool above uses 11.6% elemental iron by weight for ferrous gluconate. Depending on your ferrous gluconate product (which may use 12%), you may see slight derivations from the labeled strength but should be within a few milligrams.

Comparing Ferrous Sulfate And Ferrous Gluconate

Most studies suggest that ferrous sulfate and ferrous gluconate have essentially the same bioavailability (or rate and extent of absorption) and therefore, one is not typically preferred over the other as long as you are getting the appropriate amount of elemental iron.

Similarly, the guidelines 'Iron Deficiency Anemia: Evaluation and Management', published in the journal American Family Physician, don't recommend one over the other. The focus should be on getting the amount of elemental iron your doctor recommends.

Nevertheless, some studies suggest that ferrous gluconate may be better tolerated in terms of adverse gastrointestinal effects.

One study, published in Scientia Pharmaceutica, reported that ferrous gluconate supplements dissolve slower in the stomach, which may be why they are better tolerated. The study does state that the supposed better tolerability may just be due to the fact that ferrous gluconate has less elemental iron than ferrous sulfate.

Additional Information

Below are some key points regarding taking iron supplements:

  • For maximum absorption, take iron on an empty stomach. However, if the gastrointestinal side effects are intolerable, food will likely help.
  • Taking small doses of iron multiple times per day, as opposed to one large dose, can also increase tolerability.
  • Iron supplements may cause dark stools. This is normal and generally harmless as it is just unabsorbed iron.
  • Iron is one of the most common substances involved in accidental overdoses and poisoning. Be sure to keep out of reach of children.


About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

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