I suffered kidney failure and underwent kidney transplant. At the beginning, doctors advised me not to fast, but after three years, they differed whether I should fast or not. Some doctors said I would be able to fast, while others disagreed, considering that fasting might weaken the transplanted kidney. What should I do if I did not fast as some doctors have advised me? Should I make up for the days I did not fast on, noting that I have already made expiation? May Allah reward you.
All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
As for the days you did not fast on and made expiation for, the preponderant opinion of scholars is that you are not obliged to make up for them. This is because when one carries out what he is required to do, he will be discharged from any further duty. Moreover, you thought that you would not be cured and thus gave food to the poor as expiation as commanded. So, you are not required to do anything else in this regard.
In "Al-Mughni", Ibn Qudaamah said, "If the Muslim gave food to the poor [as expiation for breaking the fast] while he had no hope of being cured, then, later he became able to fast, he would not, most probably, be required to observe makeup fast for the missed days. This is because he is discharged from liability by making the expiation which was obligatory on him, so he is not required to carry out what he has been discharged from."
As for your fasting, you must follow the opinion of the most trustworthy doctors. If they have differed as you have mentioned, then you should follow the advice of the most knowledgeable, particularly if he possesses religious prudence and piety, or gives the advice that is supported by your experience of the effect of fasting on you.
So, respected brother, you should consult the doctor whom you trust most and who is more proficient in medicine and more religious. If that doctor told you that fasting would harm you and affect your transplanted kidney, it would be permissible for you not to fast and you will be obliged to give food to one poor person for each day you do not fast on. If that doctor told you that you would be able to fast, it would be obligatory on you to fast.
Allah Knows best.