One of the murder “weapons” I use in my mysteries is arsenic. This popular poison has been used for centuries (remember the Borgias?) and before the development of forensic science, it was hard, or impossible, to detect, especially when it was administered gradually. Death from arsenic poisoning could look like flu, cholera, or heart disease. One thing helped detectives: arsenic slows down natural decomposition of the corpse so digging up a body and finding it well preserved was a good indication of arsenic poisoning. I use that one in my as yet unpublished mysteries, THE SHILL, when an over-eager nephew bumps off his rich uncle. No wonder the substance was nicknamed inheritance powder!

ARSENIC TRIOXIDE
Lab Bottle & Watchglass of Powder.
As2O3 , Arsenic (III) Oxide, Reagent & Powder. The compound is a highly toxic, known carcinogen which, as the drug Trisenox, was FDA approved in 2000 for treatment of acute promyelocytic leukemia.
White arsenic was the preferred form; it was difficult to taste when mixed with food or beverages, particularly alcoholic ones, which plays into my plots in the Roaring Twenties when cocktails came of age.
But not all arsenic deaths were murder–some were suicide and others were purely accidental. Women used it cosmetically–advertisements assured people that the stuff was perfectly safe. Can’t imagine why anyone believed that, since every household would have had arsenic around for mice/rat control.