4. Excretion of drugs

Drug excretion is the process of eliminating a drug from the body. A drug, which is either biologically active itself or a prodrug, may be excreted in its original chemical state.

There are several routes for drug elimination from the body.

The majority of drugs are eliminated by pathways that involve the kidneys or the liver.

A major characteristic of compounds excreted in urine is that they are polarized (i.e., charged) and water-soluble.

Drugs that are lipid soluble are not readily removed by the kidneys and require hepatic metabolism (e.g., phase I and phase II biotransformation reactions) to increase their water solubility for possible urinary excretion.

The greatest proportion of drug excretion occurs through the kidneys.

•The liver makes most drugs and remedies water soluble for removal via the kidneys.

Substances with a low molecular weight and not bound to plasma proteins can easily pass through the cell membranes into the tubules.

The factors affecting the rate at which the drug or remedy is excreted by the kidneys are:

•kidney disease

•pH of urine

•change in renal blood flow

•concentration of drug or remedy in plasma

•its molecular weight.

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