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Excretion of drugs
Prepared of
Prof. Abdulkader El Daibani
Excretion of drugs
Excretion of drugs
 Drugs are excreted from the body either unchanged
or as metabolites –excretory organs with the
exception of lungs excrete polar drugs more readily
than non-polar drugs .
 The kidney in the major organ for elimination of drugs
and their metabolites.
 Drug excreted in feces are polar drugs or metabolites
excreted in bile and not reabsorbed by G.I.T.
 Pulmonary excretion is imp. in excretion of gases and
vapors.
Excretion of drugs
The major mechanisms of drug excretion
are:
A) Renal excretion-3 mechanisms :
1) Glomerular filtration (G.F):
Only free drugs can be excreted in
plasma, Therefore plasma bound are not
excreted by G.F. It depends on G.F rate
and size of drugs.
2) Active tubular secretion
In proximal renal tubule. some
organic acid can be secreted and
compete for the same mechanism
which secrete uric acid.e.g. penicillin
and probenecid-the carrier
mechanism is not selective ,so
probenecid is secreted on the
expense of penicillin and thus ↑
plasma conc. of penicillin.
Excretion of drugs
3) passive tubular reabsorption
 in the proximal and distal tubules the nonionized forms of weak acid s and bases are
reabsorbed. This mechanism is pH
dependent therefore alkalization of urine
increase the excretion of weak acid e.g.
aspirin, phenobarbitone, while
acidatification of urine increase excretion
of basic drugs.e.g.amphetamine, lidocaine
Excretion of drugs
B) Biliary excretion
 Many metabolites are excreted in bile to G.I.I. the
metabolite is excreted in feces if not are metabolized
by glucuronidases in G.I.T. But if metabolized drugs
are released and reabsorbed from G.I.T.(E.H.C) into
blood and excreted by the renal system.
C) Excretion in other organs
Lipid solubility is the limiting factor.
1) Saliva : for some drugs the conc. of dug saliva is
parallel to that in plasma e.g. Rifampin.
2) Hair, nails, skin : used for detection of toxic metals
in forensic medicine( e.g. arsenic , mercury).