Things That May Invalidate Fasting

1. Eating, drinking, and having sexual intercourse.Allah says (which means), “…So now have sexual relations with them and seek that which Allah has ordained for you (offspring), and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your 'saum' (fast) till the nightfall....” (Al-Baqarah: 187)
Sheikh al-Sa’di said, “Allah has allowed eating, drinking, and having sexual intercourse in all the nights.” (Tafseer al-Sa’di, for this verse)

2. Releasing drop of semen intentionally, either due to flirting or masturbation.It is stated in the hadith narrated by Abu Hurairah radiyallahu 'anhu, The Prophet sallallahu 'alayhi wa sallam said about the condition of one who is fasting, “…One who fasts means staying away from foods, drinks, and vain desire for the sake of Me….” (Narrated by al-Bukhari and Muslim)
One who releases a drop of semen forcefully means one is not staying away from the vain desire while fasting.

3. An infuse injection.
An infusion is one of the things that invalidate fasting because its status is the same as foods.

4. Periodic menstruation or nifas.
The Prophet sallallahu 'alayhi wa sallam said, “Aren’t women having period or nifas not allowed to perform any salah and fasting?”
On the authority of Aishah radiyallahu 'anha, “Previously, women who were having period at the time of The Prophet sallallahu 'alayhi wa sallam, we (the women) were ordered to make up (replace) the fasting and not ordered to make up the salah.” (Narrated by al-Bukhari and Muslim)

5. Vomiting intentionally.
On the authority of Abu Hurairah radiyallahu 'anhu, The Prophet sallallahu 'alayhi wa sallam said, “Whoever needs to vomit then no obligation to make up (the fasting) for him, however, whoever vomits intentionally therefore he needs to make it up (his fasting).” (Narrated by Abu Daud and al-Turmudhi; considered sahih by al-Albani)