Depends on the med. Doxycycline is less effective with chronic alcohol use. Rifampin and Isoniazid can be more toxic to the liver combined with alcohol. There likely are more interactions; these should be included in the drug information sheet that accompanies the antibiotic. If in doubt, avoid alcohol. If heavy drinker, may need to ask doctor for advice regarding cutting down; stopping suddenly can be dangerous.
Yes. Alcohol can increase the metabolism of some antibiotics, causing their levels in blood and tissue to go down rapidly, so that they are not effective. In addition, alcohol impairs our immune response, so that our white blood cells can't catch and eat germs invading our body, and other white cells cannot make enough antibodies to kill infectious agents and protect us. Avoid alcohol while on them.