How does Jekyll change physically and mentally throughout the novel?
If you go to the very last part of the novel The Strange Case of Dr. Jekyll and Mr. Hyde, you will see the chapter (section) titled "Henry Jekyll's Full Statement of the Case." In this part, Jekyll essentially makes a full confession of his "sin"—his decision to move away from a life of self-respect and virtue into the deepest, darkest side of his humanity.
Jekyll's biggest gifts, his intelligence and ability in medicine, are ultimately what lead to his demise. This is because he creates a potion to transform himself into another human being at will: a wicked, wretched, almost demonic version of himself.
Perhaps the desire for change, the longing for a different route away from a life of virtue, is the first of the many changes that he will inflict upon himself.
Hence it came about that I concealed my pleasures; and that when I reached years of reflection, and began to look round me and take stock of my progress and position in the world, I stood already committed to a profound duplicity of life.
This, as per Jekyll's own words, is the first time he senses change coming his way. What is truly amazing and radical about his change is that it is not merely internal: Jekyll is willing to transform physically and even put himself in danger. He undergoes a dangerous, physical transformation that foreshadows the pains, both physical and spiritual, that it will bring:
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.
As often happens with people who long for change, the first sensations are associated with something “better” happening. This so-called “honeymoon period” is often deceiving, as it is in the case of Jekyll. He thinks that this primary change is good because he feels
…younger, lighter, happier in body…conscious of a heady recklessness, a current of disordered sensual images…a solution of the bonds of obligation, an unknown but not an innocent freedom of the soul.
As such, almost immediately, he feels the other side of the transformation beginning to manifest. And, oddly enough, he welcomes it.
I knew myself, at the first breath of this new life, to be more wicked, tenfold more wicked, sold a slave to my original evil; and the thought, in that moment, braced and delighted me like wine.
The physical changes begin as soon as the sensation of sin and debauchery starts to surface. He starts to shrink, literally, which is a metaphor for his diminishing morality, of how much he has limited his soul.
I was suddenly aware that I had lost in stature. […]my nature[…]was less robust and less developed […]And hence, as I think, it came about that Edward Hyde was so much smaller, slighter, and younger than Henry Jekyll.
More interesting still is the awareness with which Jekyll acknowledges the disgust that Hyde causes. He concedes that evil “was written broadly” in the face of Mr. Hyde. He is aware that Hyde shows “deformity” and “decay” compared to the otherwise handsome constitution of Dr. Jekyll.
And yet when I looked upon that ugly idol in the glass, I was conscious of no repugnance, rather of a leap of welcome. This, too, was myself. It seemed natural and human.
In sum, Jekyll’s own repressed morbid desires are what leads him to the creation of an alter-ego that can enact all the things that his “real” persona would never dream of doing because of social decorum. Therefore, he does undergo a complete, profound transformation that, in the end, may altogether extinguish his humanity.
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