Ampicillin is a penicillin based antibiotic that works to combat internal bacterial infections. Ampicillin works to destruct the protective walls that bacteria creates inside your body and stops new bacteria from forming. Ampicillin is a highly effective antibiotic, and is one of the top prescribed medications by doctors! Ampicillin may also be marketed as: Principen, Totacillin, Omnipen-N, Polycillin, Principen.