In chapter 10, Dr. Jekyll explains that towards the end of his life, he lost control over his ability to transform to and from Mr. Hyde. He says that he lost control because he was unable to recreate the original drug, or potion. Dr. Jekyll explains that there was a particular salt in the original potion which he eventually ran out of, and whenever he used a new supply of what he thought was the same salt, the drug was no longer effective.

Dr. Jekyll says that he "had London ransacked" trying to find the same salt again, but that his efforts were "in vain." After trying unsuccessfully to recreate the original potion with many different salts, Dr. Jekyll decided that his original potion must have been "impure," meaning that there was something in it which he did not know was in it, and the identity of which he still does not know. It was this "unknown impurity," he says, which must have "lent efficacy to the draught." In other words, it was this unknown ingredient which made the potion effective. Unable to discover what this unknown ingredient was, Dr. Jekyll was unable to recreate the original potion, and thus unable to control his transformations to and from Mr. Hyde.

Unable to control the transformations, Dr. Jekyll became more and more over-powered by Mr. Hyde. At the end of chapter 1o Dr. Jekyll accepts that he doesn't have much time left before Mr. Hyde takes over completely. He says that he will soon "forever reindue that hated personality," and that that final transformation will "bring the life of that unhappy Henry Jekyll to an end."