There are nine acts which invalidate fast:
(i) Eating and drinking
(ii) Sexual intercourse
(iii) Masturbation (Istimna) which means self abuse, resulting in ejaculation
(iv) Ascribing false things to Almighty Allah, or his Prophet or to the successors of the Holy Prophet
(v) Swallowing thick dust
(vi) Immersing one’s complete head in water
(vii) Remaining in Janabat or Haidh or Nifas till the Adhan for Fajr prayers
(viii) Enema with liquids
Details of these acts will be explained in the following articles:-
Eating and Drinking
If a person eats or drinks something intentionally, while being conscious of fasting, his fast becomes void, irrespective of whether the thing which he ate or drank was usually eaten or drunk (for example bread with water) or not (for example earth or the juice of a tree) and whether it is more or less; even if a person, who is fasting, takes the tooth brush (Miswak) out of his mouth and then puts it back into his mouth, swallowing its liquid, his fast will be void, unless the moisture in the tooth brush mixes up with the saliva,, in such a way that it may no longer be called an external wetness.
If while eating and drinking, a person realizes that it is Fajr, he should throw the food out of his mouth, and if he swallows it intentionally, his fast is void, and according to the rules which will be mentioned later, it also becomes obligatory on him to give Kaffarah.
If a person who is fasting eats or drinks something forgetfully, his fast does not become invalid.
There is no objection to an injection which anesthetizes one’s limb or is used for some other purpose being given to a person, who is observing fast, but it is better that the injections which are given as medicine or food are avoided.
If a person observing fast intentionally swallows something which remained in between his teeth, his fast is invalidated.
If a person wishes to observe a fast, it is not necessary for him to use a toothpick before the Adhan of Fajr prayers. However, if he knows that some particles of food which have remained in between his teeth, will go down into his stomach during the day, then he must clean his teeth with toothpick.
Swallowing saliva does not invalidate a fast, although it may have collected in one’s mouth owing to thoughts about sour things etc.
There is no harm in swallowing one’s phlegm or mucous from head and chest as long as it does not come up to one’s mouth. However, if it reaches one’s mouth, the obligatory precaution is that one should not swallow it.
If a person observing fast becomes so thirsty that he fears that he may die of thirst or sustain some harm or extreme hardship, he can drink as much water as would ensure that the fear is averted. However, his fast becomes invalid, and if it is the month of Ramadan, as an obligatory precaution, he should not drink more than that, and then for the rest of the day, refrain from all acts which would invalidate the fast.
Chewing food to feed a child or a bird and tasting food etc. which does not usually go down the throat, will not invalidate the fast, even if it happens to reach there inadvertently. However, if a person knows beforehand that it will reach the throat, his fast becomes void, and he should observe its qadha and it is also obligatory upon him to give Kaffarah.
A person cannot abandon fast on account of weakness. However, if his weakness is to such an extent that fasting becomes totally unbearable, there is no harm in breaking the fast.