[Pharmacokinetic process: does the site of drug action? Excretion of drugs]

The main route of excretion of drugs is the kidney. Other routes include the lungs; breast milk; sweat; tears, and genital secretions (alarming if the patient is not expecting the orange-red discoloration caused by rifampicin); bile (leading to recirculation of some compounds, for example chloramphenicol, whose inactive metabolites are reactivated by hydrolysis in the gut, morphine, rifampicin, tetracyclines, and digoxin); and saliva (sometimes used in monitoring drug concentrations in body fluids). Three processes determine the renal excretion of drugs: glomerular filtration, passive tubular reabsorption and active tubular secretion. Thus, total renal excretion = excretion by filtration + excretion by secretion - retention by reabsorption. If an active drug is metabolized mainly to inactive compounds, renal function will not greatly affect elimination of the parent compound. However, if the drug or an active metabolite is excreted unchanged via the kidneys, changes in renal function (i.e. chronic renal failure) will influence its elimination. Therefore, dose adjustment of the drug should be recommended.