Let us remember that it is in the final chapter of this story, which features Dr. Jekyll's own account of what happened, that we discover that the initial power he used was impure, which gave the potion he created its strength and miraculous powers. The first time that he tried his concoction, he explains the physical and mental results that ensued:
The most racking pangs succeeded: a grinding in the bones, deadly nausea, and a horror of the spirit that cannot be exceeded at the hour of birth or death. Then these agonies began swiftly to subside, and I came to myself as if out of a great sickness. There was something strange in my sensations, something indescribably new and, from its very novelty, incredibly sweet. I felt younger, lighter, happier in body; within I was conscious of a heady recklessness, a current of disordered sensual images running like a mill race in my fancy, a solution of the bonds of obligation, an unknown but not an innocent freedom of the soul.
Note the way in which, in spite of the physical pain that Dr. Jekyll experiences, by far the most important effect of the drug is the new "liberty" that he feels and the way that it gives him "not an innocent freedom of the soul." Even though the drug has a rejuvenating effect, it is shown that this is not a positive process, as he is overwhelmed with "sensual images" as he himself realises that he is "more wicked, tenfold more wicked" than he was in his previous existence as Dr. Jekyll.