Answered by Salman Younas
Question: How do we determine whether our illness permits us to break the fast?
Answer: I pray you are well and in the best of health.
It is permitted to break the fast when suffering from an illness if a person reasonably:
 Fears an increase in one’s illness,
 Fears a delay in recovering from one’s illness.
 Fears falling sick.
[Shurunbulali, Maraqi al-Falah; Tahtawi, Hashiyah Maraqi al-Falah]
Imam Shurunbulali defines an “illness” as that which “necessitates the changing of one’s state to unsoundness (fasad), which occurs initially internally then its effects manifest. This is whether it is an ailment in the eye or a wound or a headache…” [Imdad al-Fattah, Ibn `Abidin, Radd al-Muhtar]
“Reasonable fear” refers to fear based on reasonable surety (ghalabat al-dhann), not on mere conjecture (wahm). Such reasonable surety can be attained by:
[a] Clear signs, such that any normal individual on seeing a person with such an illness would state that they cannot fast,
[b] Past experience, whether one’s own or someone elses who had a similar sickness, or
[c] The opinion of an expert doctor regarding an individuals condition that allows him/her to reasonably conclude that they cannot fast. An individual should exercise his or her own judgment regarding the doctor’s opinion unless the doctor is an upright Muslim who understands the limits of the sacred law.
[Shurunbulali, Nur al-Idah; Tahtawi, Hashiyah Maraqi al-Falah; Ibn `Abidin, Radd al-Muhtar]
And Allah Knows Best
Checked & Approved by Faraz Rabbani