الفهرس 
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Abstract
Although many xenobiotics are able to stimulate drug metabolism, the number of drugs which have been reported to be inducers when given at therapeutic doses is extremely low.(1) Most xenobiotics are biotransformed by hepatic cytochrome P- 450 systems to products that are less potent and more excretable than their parent compounds; in fact the duration of activity of most xenobiotics is determined by the activity of the cytochrome P-450 system(2) Since metabolism of xenobiotics by cytochromes P-450 often facilitates their elimination or makes the parent compound pharmacologically inactive,inhibition of the normal metabolic pathway may increase the intensity and duration of action of xenobiotics(3-5) To produce the characteristic effects of a xenobiotic, it must be present in appropriate concentrations at its sites of action. Although obviously a function of the amount of xenobiotic administered, the concentrations attained also depend upon the extent and rate of its absorption, distribution, binding or localization in tissues, biotransformation, and excretion(6) These factors are depicted in figure (1).
1.1.1. Xenobiotic Absorption, Bioavailability:
Absorption may be described as the rate at which а xenobiotic leaves its site of administration and the extent to which this occurs.
Bioavailability is a term used to indicate the extent to which a xenobiotic reaches its site of action or a biological fluid from which the xenobiotic has access to its site of action. For example, a.