and convulsions) if the drug is suddenly withheld. The patient habituated to narcotics
and barbiturates have physiological dependence.
Section IV. FACTORS WHICH INFLUENCE DRUG ACTION
3-7. INTRODUCTION TO PHARMACOKINETICS
Pharmacokinetics deals with the absorption, distribution, metabolism
(biotransformation), and excretion of drugs. Any time a drug is administered, these
factors will directly affect the amount of drug that will arrive at the site where the drug
acts. The amount of drug at the site of action will determine both the intensity of drug
action and the length of time the drug will show its effect(s).
3-8. ABSORPTION OF DRUGS
Absorption involves the uptake of the drug by the body. Three factors affect the
absorption of a drug: its water solubility, its fat solubility, and the transport mechanisms
of the body. It is imperative that you understand that all drugs must be in solution
before they can be absorbed.
3-9. FACTORS WHICH AFFECT ABSORPTION
a. Water Solubility. All body fluids are water based. Therefore, a drug must be
soluble in water in order to be absorbed. Dissolution of the drug in aqueous (water)
solution is dependent 6n the pH of the solution and the disintegration of the drug.
(1) Disintegration. Disintegration (Figure 3-1) increases the surface area of
a drug. The speed at which a dosage form disintegrates is dependent upon the type of
solid dosage form and the manufacturing process used to make that dosage form. The
solid dosage form could be a tablet, suppository, capsule, powder, or suspension. Take
a tablet for example. The manufacturer may add starch to the tablet in order to make it
swell when it is added to water. The tablet may be a sublingual tablet that is made to
rapidly dissolve in the mouth. On the other hand, the manufacturer may compress the
contents of the tablet under great pressure so that it will slowly dissolve. Further, an
enteric coating may be applied to the tablet so that it will dissolve in the intestine. In the
case of some capsules, "tiny time capsules" systematically dissolve during a period and
prolong the effect of the drug.
MD0804
3-12
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