"When I say the word, start spraying immediately. ...They'll come Hying out at us, I expect, but it says on the sprays one good squirt will paralyse them. When they're immobilised, just throw them in this bucket."
-- Molly Weasley on Doxycide (OP6)
A poisonous plant used in some potions.
Used in Shrinking Solution and Doxycide.
"Cow" + "Bane" meaning poisonous to cattle
The Cowbane plant, Cicuta virosa is known by other names such as "Water Hemlock" and "suicide root." It is poisonous to humans and animals, although it often takes multiple doses to be fatal.
Fan Theory: The side effects of Cowbane poisoning include pain, tremors and seizures, and while not canon, it's possible that Cowbane might have been an ingredient in Voldemort's Cave Potion which Dumbledore drank just before his death. Once clue - Cowbane essence is shown as emerald green on Book of Potions - the same color as Voldemort's Cave Potion (Pm). The other possible poison candidate "Hemlock essence" is shown on Pottermore as grayish-white. Of course knowing Voldemort, it's possible he mixed both Cowbane (Water Hemlock) and Poison Hemlock together to create a super-poison and still keep the emerald color. Interestingly, both Cowbane and Hemlock essence are also ingredients in Doxycide (Pm).