Most questions about fasting are about small details. Some matters are clear, and some are discussed by scholars. It helps to separate the two.

First, the clear cases. Eating or drinking on purpose during the fasting hours breaks the fast, and so does sexual intercourse. The Qur'an allows these only until the true dawn, describing eating and drinking “until the white thread of dawn becomes distinct to you from the black thread of night. Then complete the fast until the sunset.” Menstruation or postnatal bleeding ends a woman's fast for that day, and she makes it up later.

Second, forgetfulness. Abu Hurayrah reported that the Prophet, peace be upon him, said: “If somebody eats or drinks forgetfully then he should complete his fast, for what he has eaten or drunk, has been given to him by Allah.” That is in Sahih al-Bukhari, number nineteen hundred and thirty-three. A fast is not broken by an honest mistake of this kind, so simply continue it. Swallowing water by accident while rinsing the mouth is generally treated differently from deliberate drinking; if you are unsure about your situation, ask.

Third, matters where scholars differ. One is cupping, a traditional treatment that draws blood. Ibn Abbas reported that the Prophet was cupped while fasting, in Sahih al-Bukhari, number nineteen hundred and thirty-eight. A separate report, from Thawban, says that the one who cups and the one who is cupped both break their fast, in Sunan Abi Dawud, which scholars including al-Albani graded authentic. Because of this, the Hanbali school holds that cupping breaks the fast, while the other schools hold that it does not, reading the second report differently.

Another is modern medical procedures: injections, blood tests, inhalers, eye and ear drops, and similar things that did not exist in the same form in early times. Contemporary scholars often distinguish between what nourishes the body, which they treat as breaking the fast, and what does not, but they apply this differently in specific cases.

When in doubt, ask a qualified teacher before, rather than after, so you can act with confidence. And if a fast is broken deliberately, the lesson on making up fasts explains what follows.