Hemlock Essence

Notes

This ingredient of Doxycide, a potion for knocking out the little black biting fairies called Doxies, probably does not refer to the hemlock tree (genus Tsuga), but to the Poison Hemlock or Conium maculatum, a member of the carrot family. Other names include devil's bread and poison parsley. Lethal to both humans and animals, it was once used as a form of execution for prisoners, the most famous of whom was the Greek philosopher Socrates.

The character Banquo in Shakespeare's "Macbeth" refers to it as "insane root."

Fan Theory: Some readers have suggested that Hemlock essence may be the "emerald liquid" that Voldemort left in the sea cave to guard his Horcrux. Why? Because the poison works slowly and Dumbledore says Voldemort would want to keep the thief alive, to interrogate him : "It might paralyze me, cause me to forget what I am here for, create so much pain I am distracted, or render me incapable in some other way" (HBP26). Kreacher said it made him see "terrible things" and made his "insides burn" (DH10). Those reactions are quite similar to poison hemlock:

The conium name derives from the Greek konas, meaning “vertigo” or “whirl”—both symptoms of the plant’s intoxicating poison. ...This poison disrupts the central nervous system—a small dose can cause respiratory collapse. Death can result from blockage of the neuromuscular junction caused by coniine. In practice, it eventually stops your ability to breathe, causing you to suffocate. This won’t happen right off, and drinking its tea only makes you feel drunk at first. It may take from 48 to 72  hours for the full toxic effects to manifest. Source: Slate