According to the manufacturer, ferrex 18 refers to the 18 milligrams of ferric ammonium citrate in each dose of Geritol liquid (previously called Geritol tonic). The ferric ammonium citrate is included to provide a dietary source of iron that can be dissolved in a liquid for easier consumption.
WebMD says that Geritol is intended to treat or prevent vitamin and mineral deficiencies due to poor diet or illnesses. Constipation, diarrhea or upset stomach may occur. The stool may turn black due to the high iron content of Geritol.
WebMD warns that iron-containing supplements should be kept out of reach of young children because they can cause fatal overdoses. Individuals with iron overload disorders such as hemochromatosis should not take Geritol.