What Breaks the Fast
Fasting during Ramadan (or any voluntary fast) is broken by a specific, defined set of actions, and understanding the list precisely — including what doesn't break it, which is often the more useful half — helps avoid both accidentally invalidating a fast and unnecessarily worrying over something that doesn't affect it.
Eating and drinking, intentionally and knowingly, are the clearest fast-breakers — this covers anything taken in through the mouth and swallowed deliberately, no matter how small the amount. Intimate relations between spouses during fasting hours also break the fast and, in the specific case of Ramadan, carry a more serious expiation (kaffarah) than simply making up the day later, which is one reason it's treated distinctly from the other categories. Intentional vomiting breaks the fast as well, based on a specific hadith distinguishing it from vomiting that happens involuntarily.
What does not break the fast is often the more practically useful list. Accidentally swallowing a small amount of water while rinsing the mouth or nose during wudu, without intending to drink, does not break the fast in the majority view, since the action wasn't deliberate. Involuntary vomiting — from illness, motion sickness, or similar — is treated the same way: no intent, no effect on the fast. A mistaken meal, eating or drinking because a person genuinely forgot they were fasting, is explicitly addressed in a hadith instructing the person to simply continue their fast once they remember, without penalty, treating the food as if it had been provided by Allah.
Injections that aren't nutritive — most medications, vaccines given by injection rather than IV nutrition — are generally not considered fast-breaking by most contemporary scholars, on the reasoning that they don't function as food or drink even though they enter the body. Eye drops, ear drops, and topical applications like creams are treated similarly by most rulings, though there is more variation of opinion here than on injections. Toothbrushing and mouthwash use during fasting hours are permitted but the specific concern is avoiding swallowing anything in the process. Given how much real variation exists between schools of thought on some of these finer medical cases — and how much can depend on a specific medication or procedure — anyone managing a chronic condition or taking regular medication during Ramadan is better served asking a local scholar familiar with the specific situation than relying on a general list.