Phenol injections – originally used by the Nazis as part of the Aktion T4 euthanasia program – were used as a means of individual execution by Nazi Germany during the Second World War.

The toxic effect of phenol on the central nervous system, causes sudden collapse and loss of consciousness in both humans and animals; a state of cramping precedes these symptoms because of the motor activity controlled by the central nervous system.[“Phenol”. Ullmann’s Encycl