The Grand Ayatollah Sistani answered a question about fasting and weakness.
Question: Can a person who is unable to fast in the month of Ramadhan and is weak, fast?
The Grand Ayatollah Sistani: Weakness on its own even though it may be severe cannot be the justification for not fasting, unless it causes agonizing hardship (severe hardship which is usually unbearable) which in this case, eating and drinking (only) to the necessary amount is allowed but according to Wajeb precaution, must fast the rest of the day and must fast after the holy month of Ramadhan for the lost days and Kaffarah is not Wajeb on the person.
source : Shafaqna
