ABSORPTION OF DRUGS AFTER PARENTERAL ADMINISTRATION
Intramuscular and subcutaneous injections are by far the most common means of parenteral drug administra-tion. Because of the high tissue blood flow and the abil-ity of the injected solution to diffuse laterally, drug ab-sorption generally is more rapid after intramuscular than after subcutaneous injection. Drug absorption from intramuscular and subcutaneous sites depends on the quantity and composition of the connective tissue, the capillary density, and the rate of vascular perfusion of the area. These factors can be influenced by the coin-jection of agents that alter local blood flow (e.g., vaso-constrictors or vasodilators) or by substances that de-crease tissue resistance to lateral diffusion (e.g., hyaluronidase).
Advantages of the intramuscular and subcutaneous routes include an increased reliability and precision in the drug blood level finally achieved and reasonably rapid absorption and onset of drug action. There are, however, serious disadvantages as well. Pain, tender-ness, local tissue necrosis (primarily with highly alkaline injections), microbial contamination, and nerve damage may be associated with these forms of parenteral ad-ministration.
Intravenous drug administration ensures immediate pharmacological response; problems of absorption are circumvented because the entire quantity of drug enters the vasculature directly. This route is also useful for compounds that are poorly or erratically absorbed, are extremely irritating to tissues, or are rapidly metabo-lized before or during their absorption from other sites. The rate of injection should be slow enough, however, to prevent excessively high local drug concentrations and to allow for termination of the injection if unde-sired effects appear.
A serious disadvantage of intravenous drug admin-istration becomes clearly apparent when an overdose is inadvertently given: Neither can the drug be re-moved nor its absorption retarded. Other disadvan-tages include the possibilities of embolism (particu-larly if an insoluble drug is given), introduction of bacteria, and when this route is used for prolonged pe-riods, subcutaneous tissue infiltration. The possible in-troduction of the human immunodeficiency virus (HIV) is a well-known consequence of intravenous drug administration in addicts who use contaminated needles.