Pills of white mercury were probably made of mercurous chloride (Hg2Cl2), a mercury salt also known as calomel or mercury (I) chloride, and were taken as a treatment for syphillis. They might have had some beneficial effect because mercurous chloride, although moderately toxic to humans, destroyed the syphillis spirochete.

In the early 20th century, organic arsenic compounds such as arsphenamine (trade name Salvarsan) began to replace mercurous chloride in the treatment of syphillis. These in turn were replaced by antibiotics such as penicillin after World War II.