If we read the final chapter of this fascinating story, which gives us Dr. Jekyll's own account of all of his past and the relationship between himself and Mr. Hyde, we see that once he had began to take the transforming potion, it became incredibly hard to stop it. Even when Dr. Jekyll makes a conscious choice to choose "the elderly and discontented doctor" over the evil Mr. Hyde, and embarks on a determined course of action to note take the potion, it is clear that the temptation to embrace his darker, evil side is too powerful:
For two months, however, I was true to my determination; for two months, I led a life of such severity that I had never before attained to, and enjoyed the compensations of an approving conscience. But time began at last to obliterate the freshness of my alarm; the praises of conscience began to grow into a thing of course; I began to be tortured with throes and longings, as of Hyde struggling after freedom; and at last, in a hour of moral weakness, I once again compunded and swallowed the transforming draft.
Dr. Jekyll likens his cravings for the potion to an alcoholic thirsting after drink, and we see that having experienced the evil side of his personality, this part of his identity increasingly overpowers the good side of himself. Dr. Jekyll admits that he completely underestimated the "complete moral insensibility and insensate readiness to evil" of Mr. Hyde and thus he recognises that he is fighting a losing battle.