An antidote is a substance that can counteract a form of poisoning. The term ultimately derives from the Greek term φάρμακον ἀντίδοτον (pharmakon) antidoton, "(medicine) given as a remedy". Antidotes for anticoagulants are sometimes referred to as reversal agents. Some other toxins have no known antidote. For example, the poison aconitine – a highly poisonous alkaloid derived from various aconite species – has no antidote, and as a result, is often fatal if it enters the human body in sufficient quantities.