If you wanted to be Pope (or stay Pope) in the 1400s, there was only one way to do it. Cantarella was the poison of choice for the Borgia family. But what was in the deadly poison? And why has it become infamous?
In 1503, Pope Alexander VI had a bit of a problem with the Venetian cardinal. How lucky for him that the cardinal soon died of violent diarrhea and vomiting. Alexander had his personal troops ride out to the estate of the late cardinal, collect 150,000 ducats, and drop off nine new cardinals to replace him. In this way, the Pope received not only money from the death, but an opportunity to pack the local cardinals with people loyal to himself.
The year 1503 was the last year of Pope Alexander VI's life, and so the Venetian cardinal's death was the end of a long series of happy coincidences for him. Many times people who opposed him, or were just in his way, seemed to drop down dead. The Pope maintained this was coincidence. Others maintained that it was cantarella, a poison favored by the Pope's family, the Borgias. So often did they have need of this poison that it was said that the Pope's son, Cesare Borgia, kept a little supply of it in his ring.
What was cantarella? Many people have speculated over the years. Some people think that the name implies it was cantharidin, a powder derived from blister beetles that causes blisters to form through severe chemical burns. Most historians believe that it was a compound of arsenic. Arsenic poison, if dosed right, didn't kill instantly. Its victim got sicker and sicker, over a number of months. Arsenic disrupts the cell's energy cycle, causing cells to die. They die in such a way that the nearby phagocytes cannot and do not ingest the cell, meaning that it does not get cleared away and becomes dead, foreign material inside the body. In practice, this means weakness, confusion, vomiting, diarrhea, and intestinal pain. These symptoms, at the time of the Borgias, could be any one of a dozen illnesses.
The skill in using arsenic isn't just causing death. Arsenic was a well-known rat poison even in the 1400s. The skill was in mixing and diluting the compound just enough that the poisoning wasn't immediate and obvious. One rumor regarding the Borgias indicates they used a compound that would have killed people even if the arsenic wasn't in it. Supposedly they slaughtered and disemboweled pigs, tore out their entrails, sprinkled the entrails with arsenic, and let them sit there. After some time, they squeezed the "semi-putrid" stuff from the mess, and made that into cantarella.
It was common to suspend arsenic in animal or vegetable oils, so it's not an unbelievable story. It does explain why they felt the need to give the stuff such an excessively mellifluous name. "We poison people with rotten pig juice," isn't something a Pope should say.