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a Clinically important problems have been demonstrated with these.

In order for a drug to have a systemic action following injection, it must be released from the formulation and reach the site of action in sufficient amounts and at a sufficient rate to produce the desired pharmacological effect. The various regions into which injections are given are shown in Fig. 9.17. The subcutaneous region has a good supply of capillaries, although it is generally agreed that there are few, if any, lymph vessels in muscle proper. Drugs with the correct physicochemical characteristics can diffuse through the tissue and pass across the capillary walls and thus enter the circulation via the capillary supply.

If it is assumed that drug absorption proceeds by passive diffusion of the drug, it can be considered to be a first-order process. Thus the rate of absorption is proportional to the concentration, C, of drug remaining at the injection site:

where ka is the first-order rate constant. The half-life of the absorption process is

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