Rat poison is composed of chemicals such diphacinone, bromadiolone and warfarin — all anticoagulants that cause internal bleeding. Highly toxic metal phosphides are also a common ingredient.
Metal phosphides are quick-acting poisons that can kill a rat in a few hours by attacking vital organs and the central nervous system. Zinc phosphide is used as bait while aluminum phosphide, calcium phosphide and magnesium phosphide act as fumigants. The vitamin-D forms, known as calciferols, work by increasing the calcium concentration in the rat, leading to calcification of its organs and death. Early traditional ingredients in rat poison, such as arsenic, barium and thallium, were discontinued because they were so highly toxic. Modern rat poisons are also dangerous to humans, pets and other wildlife.