In the December 2000 issue of the Archives, Dr Mark Bernhardt1 cites a case from a century ago of what would now presumably be called mycosis fungoides, and its treatment at that time with the arsenical preparation known as Fowler's solution (arsenic trioxide dissolved in potassium bicarbonate). The therapeutic effect in that case was not noted in Dr Bernhardt's update (after 3 months there was no improvement), but he goes on to remark on the irony that this treatment poisoned many people (before its toxicity was fully appreciated), and on the recent recognition in Bangladesh that wells sunk in the 1970s to provide safe drinking water were contaminated with high levels of arsenic.
Omura GA. Arsenic Therapy. Arch Dermatol. 2001;137(6):823-824. doi: