What led to the decline and fall of Henry Jekyll?

The decline and fall of Henry Jekyll was brought about by his experimentation with the drug that transformed him into his alter ego, Mr. Hyde. As Jekyll continued to indulge in this alter ego, he lost control over the transformations, leading to his eventual demise as Mr. Hyde took over completely. Additionally, Jekyll's inability to reconcile the conflicting sides of his personality and the consequences of his actions contributed to his downfall.