Pharmacokinetics: Excretion

Excretion

Drug excretion refers to the elimination of drugs from the body. Most drugs are excreted by the kidneys and leave the body through urine. Drugs can also be excreted through the lungs, ex-ocrine (sweat, salivary, or mammary) glands, skin, and intestinal tract.

Half-life = half the drug

The half-life of a drug is the time it takes for one-half of the drug to be eliminated by the body. Factors that affect a drug’s half-life include its rate of absorption, metabolism, and excretion. Know-ing how long a drug remains in the body helps determine how fre-quently it should be administered.

A drug that’s given only once is eliminated from the body al-most completely after four or five half-lives. A drug that’s adminis-tered at regular intervals, however, reaches a steady concentra-tion (or steady state) after about four or five half-lives. Steady state occurs when the rate of drug administration equals the rate of drug excretion.