Abu Said, may Allah be pleased with him, narrates: Allah's Messenger (PBUH) said: “There are three things which do not invalidate the fast: hajamat (getting blood drawn from one's body), vomiting, and ihtilam (getting ejaculation during sleep).” (Tirmidhi, Sawm 24) As is understood from the hadith (saying of the Prophet), getting blood drawn does not break the fast. Moreover, it does not invalidate the fast to clean that area with a piece of cloth wetted with alcohol, either.

As is narrated by Abu Bakr Ibn-i Abdirrahman, may Allah be pleased with him, Aisha and Ummu Salama, may Allah be pleased with them both, told his father Marwan:

“In Ramadan, Allah's Messenger (PBUH) sometimes happened to reach the time of fajr (the time of the morning prayer) as junub (necessitating full ablution) not because of a dream; he would get up and take a bath and would fast.” (Bukhari, Sawm: 22, 25; Muslim, Siyam: 76; Muwatta, Siyam: 12; Abu Dawud, Sawm: 36; Tirmidhi, Sawm: 63; Nasai, Taharah: 123)

It is permitted to start fasting or to fast as junub and it does not invalidate the fast. However, as it is not permitted to stay junub so long as to skip one of the obligatory daily prayers, one must get full ablution (ghusl) by washing one's whole body. While getting full ablution, one must wash one's mouth. One must wash one's mouth in such a way that water should not flow down the throat (as one is fasting). It is not a must to gargle.