Dentists have a very simple reason for advising their patients not to consume alcohol after an extraction. Alcohol causes vasodilatation and this may prolong bleeding. There are very few experienced dentists who haven’t had patients showing up at their clinics one day after extraction with a blood-stained pillow (sometimes only the pillow case), livid with fear (not with the loss of blood).
Even in such situations, blood loss is usually quite minimal. Actually, we should be more worried if blood does not reach the wound. Healing gets delayed and a condition called dry socket may occur. It is very painful and takes a while to recover; definitely not something you would rather have over slightly prolonged bleeding.
Dental Phobia by Chan Joon Yee