Deijou is right. Antibiotics kill off bacteria, but some is good bacteria in the bowel (antibiotics always constipate me) or the "good: bacteria in the vaginal tract that help prevent infection there. There are andropoupillas (spelling???) in a pill that doctor had my son take after a regime of antibiotics and yogurt with active enzymes (Activia, etc.) that help regulate you. Wiyth my kids went through stuff so this is from 15 years ago.