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.

The antidotes for some particular toxins are manufactured by injecting the toxin into an animal in small doses and extracting the resulting antibodies from the host animals' blood. This results in an antivenom that can be used to counteract venom produced by certain species of snakes, spiders, and other venomous animals. Some animal venoms, especially those produced by arthropods (such as certain spiders, scorpions, and bees) are only potentially lethal when they provoke allergic reactions and induce anaphylactic shock; as such, there is no "antidote" for these venoms; however anaphylactic shock can be treated (e.g. with epinephrine).

Some other toxins have no known antidote. For example, the poison batrachotoxin – a highly poisonous steroidal alkaloid derived from various poison dart frogs, certain beetles, and birds – has no antidote, and as a result, is often fatal if it enters the human body in sufficient quantities.