The World Federation One Stop Fiqh

Ruling 1696

If someone has an illness that makes him very thirsty and he cannot bear being thirsty, or it is excessively difficult for him to bear it, then fasting is not obligatory for him. However, in the second case, he must give one mudd of food to a poor person for each missed fast; and in the event that he is able to fast afterwards, it is not obligatory for him to make them up.