Shaikh Uthaimeen, may Allaah have mercy on him, was asked, “What is the scale/degree of harm according to which a fast becomes harmful for someone who is ill and after which it becomes forbidden?” He replied, “Indeed, harm is known through feeling and it maybe known by being informed of it. As for knowing it by feel then it is if an ill person feels that the fast is harming him and making his pain worse and delaying his recovery. And as for [recognising that] by being informed then it is if a knowledgeable, trustworthy, doctor tells him that [it is harmful], and it is not a condition that the doctor be a Muslim.”

Ash-Sharh al-Mumti, vol. 6, pp. 328-329.