Please consult your physician before taking any medication. Generic Name: Ampicillin Capsules (AM-pi-SIL-in) Brand Names: Principen, Pentrexil Ampicillin is used for:Treating infections caused by certain types of bacteria. Ampicillin is a penicillin antibiotic. It works by killing sensitive bacteria by interfering with formation of the bacterias cell wall while it is growing. This weakens the cell wall and it ruptures, resulting in the death of the bacteria. Ampicillin may also be used for purposes other than those listed in this medication guide.