Fasting: Fasting When Personal Certainty of Harm Conflicts with a Doctor's Advice
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1711
Someone who knows fasting will not cause him significant harm – even though a doctor says it is harmful for him to fast – must fast. And someone who is certain or supposes that fasting will cause him significant harm – even though a doctor says it will not harm him – is not obliged to fast.