Strychnine (pronounced strik'-neen (British) or strik'-nine (U.S.)) is a very toxic, colourless crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as rodents. It works by inhibiting the action of glycine in the nervous system, causing muscular convulsions and eventually asphyxiation or sheer exhaustion. The most common source of exposure is from the nux vomica tree.

Strychnine is very bitter-tasting; in fact, it is the bitterest substance in the world! Its taste is detectable in concentrations as small as 1 part per million.

Strychnine poisoning can be fatal to humans, by inhalation, swallowing or skin contact. It produces some of the most dramatic, terrifying, best known, and painful symptoms imaginable. For this reason, strychnine poisoning is often used in literature and film. The approximate lethal dose is extremely small, less than 0.2mg/kg.

Ten to twenty minutes after exposure, every muscle in the body will start to simultaneously contract, starting with the head and neck. The spasms then spread to every muscle in the body, with nearly continuous convulsions. They get worse at the slightest stimulus. They progress, increasing in intensity and frequency until the backbone arches continually. Death comes from asphyxiation caused by paralysis of the brain's breathing apparatus, or by exhaustion from the convulsions. At that time, the body "freezes," even in the middle of a convulsion. Rigor mortis sets in immediately, with the eyes left wide open.

Treatment involves giving depressants to control the convulsions. If the patient lives 24 hours, recovery is probable.