If a person becomes exhausted due to illness, finds it difficult to complete the fast of that day, or thinks that continuing the fast would worsen their current condition—in short, if they fear that “the illness may increase or be prolonged”—then they may break their fast and later make up for the missed days.
In cases of sudden or temporary illness, this is contingent upon the individual’s own judgment. However, in the case of ongoing chronic conditions, one should act in accordance with the advice of a qualified physician. In some instances, the patient’s own experience may also be sufficient in making this determination. For example, a person with diabetes who is familiar with their condition may be able to assess, to a reasonable extent, how fasting affects them; thus, their personal evaluation may suffice.
If the effects of the illness persist for that day and for several days thereafter, the person may refrain from fasting during this period. Particularly where medication prescribed upon the advice of a physician is involved, this may continue until the body has recovered.