An introduction to principles of veterinary clinical pharmacology: the absorption of drugs

An introduction to principles of veterinary clinical pharmacology: the absorption of drugs
Peer reviewed


In order to act and produce effects, unless it acts topically at the site of action, a drug must be absorbed into the blood. The rate of absorption depends upon the route of administration and physicochemical characteristics of the drug. The physico-chemical properties that affect the absorption of a compound are lipid and water solubility, degree of ionization, chemical stability of the drug and molecular weight. A substance will probably be well absorbed if it is lipid-soluble and at the same time also soluble to some extent in water, if it is not completely ionized, and, finally, if its molecular weight is not excessively high (Keberle, 1971) . Absorption from all sites of administration is dependent upon drug solubility. Drugs given in solution are more rapidly absorbed than those given in solid form. For those administered in solid dosage form, the rate of dissolution becomes the limiting factor in their absorption. The concentration of a drug also influences its rate of absorption. Other factors are the circulation to the site of absorption and the area of the absorbing surface to which the drug is exposed. Since the absorbing surface is determined largely by the route of administration, it is evident that the rate of drug absorption is principally determined by this factor. All drugs not injected intravenously present potential bioavailability problems.

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