- Peninei Halakha
The prophets and the Sages instituted the fasts for healthy people, not for sick people. In this, Yom Kippur differs from all other fasts. On Yom Kippur, even sick people must fast, as it is a Torah command. Only those whose lives may be at risk if they fast are exempt, since saving a life overrides the Torah’s commandments. On the other fasts, however, which were instituted by the Sages, anyone who is sick is exempt from fasting, even if his condition is not life-threatening.
In general, one is considered sick if his pain or weakness prevents him from continuing his normal routine of life, forcing him to lie down. For example, those who suffer from the flu, angina, or a high fever do not need to fast.
Almost everyone experiences headaches and weakness on fast days, and most people find it easier to spend the day in bed than to continue to function normally. Sometimes, one who is fasting feels even worse than one who has the flu. Nonetheless, this is not considered a sickness, but rather the natural effects of fasting, which will pass within a few hours after the fast is over. Therefore, only one who needs to lie down because of an illness is exempt from fasting. One who is suffering from the fast itself, however, must continue to fast even if he prefers to lie down in bed due to his weakness. One is only exempt from the fast if he becomes so weak from fasting that he actually becomes ill.
In addition, anyone who knows that fasting may cause him to become sick does not need to fast. For example, one who suffers from an active ulcer or severe migraines is exempt from fasting, because it is liable to precipitate his illness. Similarly, a weak person who knows that there is a good chance that he will become ill if he does not eat is exempt from fasting. Diabetics who need to take insulin do not need to fast and are even exempt from fasting on Yom Kippur. Those who have kidney stones and thus need to drink large quantities of water are exempt from fasting. One who has high blood pressure is not considered sick and should fast, unless his doctor instructs him otherwise. Whenever one is in doubt, one should consult a God-fearing doctor.
One who is exempt from fasting because of an illness may, le-khatĥila, eat a full meal and drink as much as he needs starting from the morning, but it is inappropriate for him to indulge in delicacies. He need not ration his food into very small quantities, as a sick person might be instructed to do on Yom Kippur, because sick people are simply not obligated to fast on fast days of rabbinic provenance. It is only on Yom Kippur, when fasting is mandated by Torah law that even sick people must fast, that the Sages ruled stringently that those who are dangerously ill and need to eat must do so in quantities smaller than the threshold of punishment, if possible, to avoid breaking the fast.
It is also important to note that sick people who need to take medication regularly, like one who has started a regimen of antibiotics or one who suffers from a chronic disease, must continue taking their medication even on a fast day. If possible, one should swallow the medication without water. It should be noted that almost no medication, including antibiotics, does any harm to one who swallows it without water. One who cannot swallow pills without water should add something bitter to the water to the point that it becomes undrinkable, and then use it to swallow the pill.
 Regarding the various illnesses, I enlisted the aid of R. Prof. Avraham Steinberg, author of Encyclopedia of Jewish Medical Ethics. Some say that there is a difference between Tisha Be-Av and the other, minor fast days. On Tisha Be-Av, only one who is actually sick or liable to become sick because of the fast is exempt. On the minor fasts, however, even one who is in great pain – significantly more than other people – is exempt from fasting. This is derived from the law of pregnant and nursing women, who are not considered sick and are thus obligated to fast on Tisha Be-Av and exempt from the minor fasts. Accordingly, ahs 550:1 states that one who is weak should not be stringent and fast on the minor fasts; he must fast only on Tisha Be-Av. Kaf Ha-ĥayim 550:6, 554:31 concurs. However, many poskim do not make such a distinction, and we can explain their reasoning as follows: the Sages exempt only pregnant and nursing women from the minor fasts because the fetus or baby needs nourishment, but those who are suffering but not actually sick remain obligated. Either way, it seems that in cases of doubt, one may be more lenient on the minor fasts than on Tisha Be-Av.
 Less than the threshold of punishment (paĥot mi-keshi’ur) means drinking less than melo lugmav (a cheekful) and eating less than the bulk of a date (ke-kotevet) within nine minutes. See below ch. 10, n. 2, where we cite more stringent opinions, mainly regarding Tisha Be-Av, though most poskim follow the more lenient view.