Shaikh Ibn ul-Uthaymeen rahimahullah wrote:
“The sick person has different conditions:

1. The sickness does not affect the fast, like light cold, or light headache, or toothache and what resembles that. So it not allowed for this one to break the fast, even though some Scholars say it is allowed due to the generality of the verse: “whoever is sick” [2:185]. However we say: this ruling is connected to its reason and that if breaking the fast is more gentle, then we say: break the fast. As for if it does not affect, then it is not allowed to break the fast and it is obligatory to fast.

2. If fasting is difficult but does not harm him, then it is disliked for him to fast and it is Sunnah for him to break his fast.

3. If fasting is difficult for him and harms him, like a man who is afflicted with kidney disease or diabetes and what resembles that. Then fasting upon him is haram”

[Sharh Mumti’ (6/341)]

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan