Physically, Jekyll starts out as a typically prosperous doctor, mostly appearing to be a good man, though with a shadow. He is described as "a large, well-made, smooth-faced man of fifty, with something of a slyish cast perhaps, but every mark of capacity and kindness."
But as Hyde more and more takes hold of him, he changes physically: "I became, in my own person, a creature eaten up and emptied by fever, languidly weak both in body and mind, and solely occupied by one thought: the horror of my other self."
Mentally, Dr. Jekyll also deteriorates, as like an alcoholic or drug addict, he is less and less able to control his inner Mr. Hyde. He is torn between his desire to become Hyde, and through him, to indulge his darkest impulses, and his increasing loathing and dread of what Mr. Hyde is and does. Over time, he becomes increasingly aware that the compromise he has devised by dividing himself into two people has been a terrible mistake. For some months, he is able to resist turning into Hyde, but again the desire overwhelms him. Finally, he completely loses control of Mr. Hyde and at that point he commits suicide.
First, an almost invisible change: he starts planning for possible negative fates. That is to say, he makes his will and gives it to Utterson. He knows things might go wrong. A second change: he goes ahead with his experimentation anyway. When Utterson sees Jekyll in Part 2, Jekyll looks "deathly sick" and carries himself with a "feverish manner." Both of these are changes in body (and the second a change in mind). In Part 3, Jekyll loses control of his fate (also body and mind, to Hyde); this means he is now at the mercy of others. Finally, though, he becomes responsible: forcing himself into a situation where he'd be punished for his crimes (as Hyde).
Not only did he change mentally but also physically and the changes can be seen due to the fact that his character is becoming much easier to understand and it seems as if though Jekyll acts like a drugee and Mr.Hyde is the drug which gives power to those things he can do when he is not bounded by law