Hogwarts students learn about bezoars in their first year potions class. They’re stones from the stomach of a goat that work as an antidote to most poisons. In book 6, Harry saves Ron’s life by giving him a bezoar after he accidentally drinks poisoned mead in Slughorn’s office.
J.K .Rowling did not make this up: bezoars really exist. They can be found in the stomach or intestines of various animals (including goats, but also humans, elephants , or llamas ), and are composed of undigested hairs (trichobezoar) or nondigestible food material such as cellulose (phytobezoar).
In the 16th century, bezoars (especially from goats and cows) were believed to be universal antidotes. They were also quite rare, and heavily sought after by the rich. King Charles IX of France (1550-1574) was excited when he acquired a bezoar from Spain. He showed it to court physician Ambroise Paré (1510-1590), asking if there was any antidote quite as good as a bezoar like that. Paré told the king that he did not believe there was such a thing as a universal antidote, because there are so many different types of poisons. He suggested an experiment to prove that the bezoar would not work.
Paré asked if there were any prisoners scheduled for hanging. As it happened, a cook was just put on death row for stealing some silverware. The king made the cook a proposition: he could be hanged as planned, a quick and sure death, or he could be poisoned, at which time he would also be given a bezoar. If the bezoar successfully blocked the poison, he would be allowed to keep his life. Naturally, the cook happily took this chance and agreed to be experimented on.
He was given poison by the court pharmacist, immediately followed by the bezoar. The cook writhed in agony, threw up, was given water when he asked for it, but nothing seemed to help him. When Paré visited him shortly after, he was bleeding from all orifices. Paré gave him oil to drink, but that didn’t help either, and after seven painful hours the cook died. Seven hours during which he undoubtedly wished he had just chosen the hanging…
Paré thought this the ultimate proof that bezoars are not a universal antidote, and up until today this is considered the key experiment in this matter, but Charles IX didn’t buy it. He just thought he had probably gotten his hands on a faulty bezoar…
A more recent bezoar experiment shed some light on how the bezoar myth came to be. Gustaf Arrhenius and Andrew A. Benson of the Scripps Institution of Oceanography showed that at least certain types of bezoars can bind arsenic. If arsenic was a common poison in the Middle Ages and Renaissance, that suggests that sometimes bezoars might have worked.
If we go back to the Harry Potter books, this suggests that Professor Slughorn’s mead might have been poisoned with arsenic, because the bezoar really did help.
Today, cow bezoars are still used in traditional Chinese medicine. Not as a universal antidote, but for example to cure gum pain. With the westernization of Chinese science, some studies are now looking for the active compounds of bezoars.
(Read more about Paré (including his own account of the cook’s poisoning) in Paré’s Law: The Second Law of Toxicology. He was way ahead of his time, and pretty awesome. I think he also singlehandedly increased the average life expectancy for that period by several years by making it to 80!)