the poisonous secretion of an animal, produced by specialized glands that are often associated with spines, teeth, stings, or other piercing devices. The venom apparatus may be primarily for killing or paralyzing prey or may be a purely defensive adaptation. Some venoms also function as digestive fluids. The venom poisoning of humans is primarily a problem of rural tropical regions, though it occurs worldwide. Many thousands of human deaths due to venom poisoning occur each year.

Most venoms injure humans only when introduced into the skin or deeper tissues, usually through a sting or bite. Venoms are mixtures of toxic enzymes and various other proteins that act on the body in different ways. Neurotoxin venoms act on the brain and nervous system and can cause either nervous excitation (characterized by such symptoms as muscle cramps, twitching, vomiting, and convulsions) or nervous depression (with such symptoms as paralysis and weakening or arrest of respiration and heartbeat). Hemotoxins affect the blood or blood vessels: some destroy the lining of the smaller blood vessels and allow blood to seep into the tissues, producing local or widespread hemorrhages, while others render the blood less coagulable or cause abnormally rapid clotting, leading to circulatory collapse that can be fatal. Still other venoms produce the symptoms of an allergic reaction, resulting in wheals, blisters, and violent inflammation, often followed by death of tissue and muscle spasms.

Most major animal phyla contain venomous species, but relatively few come into harmful contact with humans. These few include certain snakes (e.g., cobras, mambas, vipers, pit vipers, coral snakes, and rattlesnakes); certain fishes (e.g., stingrays, weevers, spiny sharks, scorpion fish, ratfish, and certain catfish); a few lizards (Gila monster); some scorpions and several spiders (e.g., the black widow and brown recluse); some social insects (e.g., the bee, wasp, and certain ants); and various marine invertebrates, including some sea anemones, fire corals, jellyfish, cone shells, and sea urchins. Snakes and spiders inject venom into their victims with their fangs; fishes use venomous spines and scorpions and many insects use stings.

Venom attacks can range in severity from a simple localized inflammation of the skin to almost immediate death, depending on the animal involved and the potency and mode of action of its venom. An attack's severity also depends on the victim's age (children are more severely affected than are adults) and the location of the injury (a venom wound on an arm or leg is usually less serious than a similar one on the head or trunk). See also poison.