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Figure 2B: The possible sites of drug elimination during

Figure 2B source:

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2002 Guide to Psychotropic Drug Interactions

Decreased rates of drug absorption are clinically
important for drugs that are given as a single dose in situations requiring
rapid onset of action, for example with analgesics or hypnotics. Therefore drug
concentration can also be affected by the product formulation. As with drugs
that undergo first-pass metabolism, drugs that are incompletely absorbed after
oral administration will have much higher blood concentration when they are
administered intravenously. The intramuscular and subcutaneous administration
will yield lower peak blood concentrations than those observed following
intravenous administration. On the other hand
the rate of absorption is usually unimportant for compounds which are given in
multiple dose regimen to achieve a desired therapeutic plasma concentration,
however while the drug may be completely absorbed, this may occur so slowly
that effective blood levels are never reached, or the rate of onset may be too
slow when prompt action is required.     


 2.2 Serum: Tissue Concentration

 In order for a
drug to exert its characteristic effect, it must reach its site of action. The
site of action is where a drug acts to initiate the chain of events leading to
an effect. Though it would be useful to monitor the concentration of drug at
these sites, receptor sites of drugs are generally inaccessible to our
observations or are widely distributed in the body, therefore direct
measurement of drug concentrations at these sites is not practical (Rizk et al. 2017). For example, the receptor sites for digoxin are
thought to be within the myocardium. Obviously, we cannot directly sample drug
concentration in this tissue. However, we can measure drug concentration in the
blood or serum, urine, saliva, and other easily sampled fluids (Peng and Chiou 1990). Unless a drug is purposely
administered to produce it is effect locally, or is injected directly into the
bloodstream, getting to its site of action involves two separate
processes.  The first of these is
absorption (we covered in 2.2 section of this chapter). The second process is distribution.
Distribution is defined as the dispersion of a drug throughout the body so it
is the process by which the drug goes from vascular space to extravascular
space. Extravascular spaces include different compartments in the body, for
example, interstial space, fat, muscles and bone etc., so these are the spaces
or volumes that should be considered when we calculate drug concentration (Figure 4)


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