Carbonyl Iron. Pure form of Iron. No. 1 doctor recommended iron supplement. Designed to be gentle to your stomach. 45 mg of Elemental Iron per caplet - 250% of Daily Value (Note: The 45 mg of Elemental Iron per Feosol Caplet is the same amount of Elemental Iron as in 225 mg of Ferrous Sulfate USP.) FE - Brand of Iron most doctor recommended. In an easy to carry bottle. Advanced Formula Feosol Caplets: Most ordinary iron products contain iron salts that can upset your stomach. Feosol Caplets are different. They contain pure iron micro particles called carbonyl iron. This advanced formula is specially designed to be well absorbed by your body and gentle on your stomach.