People long ago recognized that depending on the dose, arsenic could either treat an illness or be used as a poison to cause death. Its medicinal use to treat syphilis and amebic dysentery ended with the introduction of penicillin and other antibiotics in the twentieth century. Arsenic-based compounds are currently used to treat some forms of cancer. As a poison, arsenic trioxide (As2O3) has several desirable qualities: it looks like sugar and is tasteless, and it only takes about a tenth of gram to kill someone. While its use as a human poison has greatly declined, arsenic is still used as an herbicide, particularly in growing cotton, and as a wood preservative. Arsenic poisoning from well water remains a serious worldwide human health concern.

(Image: Native arsenic with quartz and calcite, from Ste. Marie-aux-mines, Alsace, France)