Gentamicin is not given orally because:-

AMINOGLYCOSIDES These include streptomycin, gentamicin, kanamycin, tobramycin, amikacin, sisomicin, netilmicin, neomycin and framycetin. These are not absorbed orally and do not cross blood brain barrier. These are excreted primarily by glomerular filtration and the dose should be decreased in renal insufficiency. These drugs exhibit CDK and have prolonged PAE, therefore are administered as single daily dose. Aminoglycosides are bactericidal inhibitors of protein synthesis. Their penetration across the cell wall is dependent on the oxygen dependent transpo, therefore these drugs are inactive against anaerobes. Resistance to these drugs develops due to the formation of inactivating enzymes