The unique story of ketamine began in 1956 with the synthesis of the drug, phencyclidine. Found to be a remarkable anesthetic for monkeys, phencyclidine was then used on humans after numerous animal testings. Unfortunately, the side effects that researchers observed in some patients were concerning. Instead of waking peacefully from anesthesia, some patients experienced acute emergence delirium lasting hours after their operation was over. They described feeling sensory deprived and the loss of sensations in their limbs. Further studies concluded that phencyclidine produced a sensory deprivation syndrome.