Bezoar stone in a gold filigree case

In the Halfblood Prince, Harry saves his best friend, Ron Weasley’s life by shoving a bezoar down his throat, to counteract the effects of poisoned mead. Bezoars, a mass of undigested fibre formed in the stomach of animals and believed to be antidotes to poison, were first introduced into medieval Europe by Arabic physicians.

Though doubts were cast over their effectiveness, they were used well into the 18th century, probably because just ingesting the stone would have made you sick.