Cadaverine

Cadaverine is obtained from the bodies of drowned dead, devourers, drowners, alghouls and graveirs.

Source[]

In Chapter I, an "Old townswoman" will tell Geralt a tale about drowned dead in exchange for food. This conversation results in journal entries for Drowners (not Drowned Dead), Drowner brain tissue, Cadaverine, and Ginatz's acid.
Swamp Monsters
The Tome of Fear and Loathing, volume I
The Tome of Fear and Loathing, volume II

Geralt can loot the bodies of drowners for cadaverine and ginatz's acid before he has a Bestiary entry for drowners or ingredient entries for those items. However, drowner remains will not contain drowner brain tissue until Geralt has the journal entries.
Geralt has a conversation about the poisonous nature of cadaverine with a peasant woman in the village of Murky Waters in Chapter IV.
Although this item does not appear in The Witcher 3: Wild Hunt, it is mentioned by Geralt in the quest Contract: The Merry Widow, helping Geralt to find the ghoul.