How long do antibiotics take to work?

When you are taking antibiotics, it does take several days for the medicine to kill enough of the bacteria inside your body so that you actually feel the difference. I always tell my patients that if you don't notice a real improvement on Day 4, it's time to call the doctor back. Usually, by the fourth day, enough of the bacteria are dead for you to feel almost back to yourself. If you still feel lousy, the doctor probably needs to switch antibiotics to try and find another one which will kill them off.