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How does Dr. Jekyll become Mr. Hyde in Robert Louis Stevenson’s novella?

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Hyde, Mr.; Jekyll, Dr. — Mr. Hyde Mr. Hyde (left) and Dr. Jekyll, both as portrayed by Fredric March, 1931.

In Robert Louis Stevenson's novella, The Strange Case of Dr. Jekyll and Mr. Hyde, Dr. Jekyll transforms into Mr. Hyde through a secret potion he developed. This potion was designed to separate the good and evil aspects of his personality.1

Initially, Dr. Jekyll could easily revert from his Hyde persona back to his respectable self. However, over time, he found himself involuntarily transforming into Mr. Hyde without the aid of the potion. Eventually, he ran out of the necessary ingredients for the drug due to an impurity in the original supply. In his final moments before permanently becoming Hyde, Jekyll wrote a confession.1