In other cases, respiratory infections have resolved despite seemingly inappropriate antibiotic therapy. The etiology of an infection is often unknown when therapy is initiated. The pathogens causing urinary-tract or sinus infections can usually be readily identified, but respiratory infections are caused by so many different organisms that it is impractical and unnecessary to test for them all. When planning empiric therapy, one need be concerned only that the drug selected is active against organisms commonly encountered. Among the gram-negative aerobes, Escherichia coli and Proteus mirabilis are usually susceptible, but Haemophilus influenzae, Bordetella pertussis, and Proteus vulgaris are resistant.
Anaerobes are also resistant.
Because of its poor activity against H. influenzae, cephalexin is not generally employed for upper respiratory or sinus infections. Cephalexin is well absorbed by the gastrointestinal tract and reaches peak activity levels very rapidly. It is not metabolized by the body but is excreted unchanged in active form by glomerular filtration and tubular secretion. It is thus present in high concentrations in the urinary tract. Some patients experience hypersensitivity skin reactions or gastrointestinal symptoms such as diarrhea and vomiting.