Excipient (pharmacologically inactive substance)
What is it?
Ferrous oxide (FeO), or iron (II) oxide, is an iron oxide and exists as a black powder. It is not rust (ferric oxide). Iron(II) oxide makes up part of the earth's mantle. Synthetic iron oxide is approved for used as a pigment in certain foods, cosmetics, contact lenses and drugs.
 FDA. For Industry. Summary of Color Additives for Use in the United States in Foods, Drugs, Cosmetics, and Medical Devices. Accessed April 1, 2014. http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=73.1200