Knowledge of important pharmacokinetic parameters of a drug, such as its half-life in plasma and its distribution and body clearance, is helpful in understanding the time course of the plasma concentration of a drug as a function of dosage. Appropriate doses of drugs with a slow rate of elimination, such as cefonicid, may be administered less frequently than other drugs. Where substantial elimination occurs by renal excretion of unchanged drug, e.g., for all of the cephalosporins except cefoperazone and cefotaxime, modification of dosage is necessary when renal insufficiency is present. Such dosage modification can be estimated by noting proportional changes from normal in the glomerular filtration rate (estimated or measured creatinine clearance). Dosage modifications include changes in the amount of drug given, the interval between doses, or both.