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Chapter: Essential pharmacology : Pharmacokinetics; Metabolism Excretion Of Drugs, Kinetics Of Elimination

Excretion is the passage out of systemically absorbed drug. Drugs and their metabolites are excreted in:



Excretion is the passage out of systemically absorbed drug. Drugs and their metabolites are excreted in:




Through the kidney. It is the most important channel of excretion for majority of drugs (see below).




Apart from the unabsorbed fraction, most of the drug present in faeces is derived from bile. Liver actively transports into bile organic acids (especially drug glucuronides by OATP and MRP2), organic bases (by OCT), other lipophilic drugs (by Pgp) and steroids by separate nonspecific active transport mechanisms. Relatively larger molecules (MW > 300) are preferentially eliminated in the bile. Most of the drug, including that released by deconjugation of glucuronides by bacteria in intestines is reabsorbed (enterohepatic cycling) and ultimate excretion occurs in urine. Drugs that attain high concentrations in bile are erythromycin, ampicillin, rifampin, tetracycline, oral contraceptives, phenolphthalein.

Certain drugs are excreted directly in colon, e.g. anthracene purgatives, heavy metals.


Exhaled Air


Gases and volatile liquids (general anaesthetics, paraldehyde, alcohol) are eliminated by lungs, irrespective of their lipid solubility. Alveolar transfer of the gas/vapour depends on its partial pressure in the blood. Lungs also serve to trap and extrude any particulate matter injected i.v.


Saliva And Sweat


These are of minor importance for drug excretion. Lithium, pot. iodide, rifampin and heavy metals are present in these secretions in significant amounts. Most of the saliva along with the drug in it, is swallowed and meets the same fate as orally taken drug.




The excretion of drug in milk is not important for the mother, but the suckling infant inadvertently receives the drug. Most drugs enter breast milk by passive diffusion. As such, more lipid soluble and less protein bound drugs cross better. Milk has a lower pH (7.0) than plasma, basic drugs are somewhat more concentrated in it. However, the total amount of drug reaching the infant through breast feeding is generally small and majority of drugs can be given to lactating mothers without ill effects on the infant. Nevertheless, it is advisable to administer any drug to a lactating woman only when essential. Drugs that are contraindicated during breast feeding or need special caution are given in Appendix3 at the end of the book.


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