Table Penicillins Drug Route Natural penicillins Penicillin G (parenteral / aqueous) Parenteral Penicillin G benzathine Parenteral Penicillin G procaine Parenteral Penicillin V potassium Oral Aminopenicillins Ampicillin Oral or parenteral Ampicillin / sulbactam Parenteral Amoxicillin Oral Amoxicillin / clavulanate Oral Penicillinase-resistant penicillins Dicloxacillin Oral Nafcillin Oral or parenteral Oxacillin Oral or parenteral Broad-spectrum (antipseudomonal) penicillins Carbenicillin Oral Piperacillin Parenteral Piperacillin / tazobactam Parenteral Ticarcillin Parenteral Ticarcillin plus clavulanate Parenteral Resistance Some bacteria produce beta-lactamases, which inactivate beta-lactam antibiotics; this effect can be blocked by adding a beta-lactamase inhibitor.