There are several cases on record of pigmentation following the local application of iron-containing medicinal substances. One may well take warning from these that the use of certain agents is liable to be followed by discoloration, which may be permanent.
REPORT OF CASE
Miss G. H., a robust woman, aged 33, with blue eyes, dark hair and brunette skin of fine texture, contracted impetigo, which commenced with "blisters" on her neck, June 13, 1936. For five days she applied tincture of iodine several times a day, without benefit. Then, at the suggestion of a neighbor, she made up a strong solution of copperas, which is ferrous sulfate, FeSO47H2O, a translucent, greenish crystalline substance known also as green vitriol. She did not measure the strength, simply dissolving several teaspoonfuls in a glass of water. She applied the solution by "dabbing it on several times a day for
Sutton RL. PIGMENTATION OF THE SKIN DUE TO IRON (COPPERAS) APPLIED LOCALLY. JAMA. 1937;108(2):112-113. doi:10.1001/jama.1937.92780020002009a