The administration of an antibiotic depends largely on the clinical picture, the condition of the patient, and the availability. Most antibiotics can be given orally and parenterally. Modern beta-lactams (many penicillins and cephalosporins) and aminoglycosides have to be injected because they are not absorbed in the intestine. Only a few substances (phenoxymethylpenicillin, norfloxacin, oral esters of ampicillin) are not available in parenteral form. Antibiotics can be divided into several groups according to their absorption.