You did not provide the excerpt but in the end Jekyll was not able to permanently resist. It was implied that he was actually Hyde deep inside and that Hyde is the one who will overtake and he won't be able to come out again. In the end he dies as Hyde and people find out what happened out of letters that he sent while he was sane.
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 an hour of moral weakness, I once again compounded and swallowed the transforming draught.
D. <span>Jekyll will not be able to permanently resist becoming Hyde.</span>
