What should a Muslim do if a doctor advises them not to fast?

TL;DR: If a qualified doctor advises a Muslim not to fast due to a medical condition or illness, Islamic law permits them to break their fast and make up for the missed days later or provide a compensatory payment (fidyah) if unable to fast. This exemption is based on clear evidences from the Quran, hadith, and scholarly consensus to alleviate hardship and prioritize health. Exemption from Fasting Due to Illness The Quran and Sunnah provide explicit exemptions from obligatory fasting for those who face undue hardship or harm due to illness or medical conditions. This principle is based on the following evidences: "But if any of you is ill or on a journey, the same number (of days which one did not fast must be made up) from other days." (Quran 2:185) The Prophet (saw) said: "Whoever is sick or traveling, then let him break the fast and make up for the missed days later." (Bukhari) There is a scholarly consensus (ijma') among Muslim jurists that individuals who are genuinely ill or face significant harm from fasting are exempt from the obligation to fast during their illness. This exemption is a manifestation of the Islamic principle of removing hardship and promoting ease. Imam al-Nawawi (d. 1277 CE) stated: "If fasting causes harm or exacerbates an illness, it is not required, based on the verse and the hadith." Ibn Qudamah (d. 1223 CE) said: "The ill person who fears harm from fasting is permitted to break the fast, whether the illness is chronic or temporary." Conditions for the Exemption - The illness or medical condition must be confirmed by a qualified and trustworthy physician. - The illness should cause significant harm or difficulty if the person were to fast. - The exemption applies only for the duration of the illness. Once recovered, the missed fasts must be made up. Compensatory Payment (Fidyah) - If an individual is unable to make up the missed fasts due to a chronic or permanent illness, they are required to provide a compensatory payment (fidyah) for each missed day. - The fidyah typically involves providing a day's worth of sustenance to a needy person, as determined by local authorities or scholars. Conclusion In Islam, preserving one's health and well-being is a crucial principle. If a qualified doctor advises a Muslim not to fast due to a medical condition or illness, they are permitted to break their fast and make up for the missed days later or provide a compensatory payment if unable to fast. This exemption reflects the compassionate nature of Islamic law, which aims to alleviate hardship and prioritize the well-being of individuals.