-Most common drug administration is through the mouth (per os or P.O); passes through
the GI tract, safest, cheapest, and convenient
-Another way is rectal administration.
-Several factors can influence absorption of drugs from the G.I tract, which can greatly
alter the rate of drug accumulation, its concentration and its duration at the sites of action.
-Most drugs are thought to penetrate the G.I mucosa by a process of passive diffusion,
which in turn is limited mostly by the drug’s lipid-solubility.
-Many drugs are weak bases or weak acids, and this alkalinity or acidity often results in
their being ionized.
-Such ionization reduces their solubility in cellular membranes, because proteins
embedded in these membranes have a mixture of positive and negative charges that tend
to repel charged particles.
-To facilitate their absorption, such drugs are commonly administered in the form of a
salt, a compound formed by the combination of a base and acid.
-Drugs may exist as both the nonionized and ionized species.
-The nonionized form is more lipid-soluble than the ionized form, the proportion of non-
ionized to ionized drug molecules present in a given area is important for drug absorption
-The pH of the local area determines the ratio of ionized to nonionized rug in that area.
-Weak acids, like aspirin, are less ionized in an acid medium and are therefore more
lipid-soluble through the stomach, with a pH of less than 3.
-Alkaloids like heroin, morphine, ad cocaine are poorly absorbed from the stomach. The
small intestine is slightly alkaline which favours the absorption of weak bases.
-This explains why the greater surface area of small intestine.
-People get intoxicated faster when they drink carbonated alcoholic drinks because it’s
more alkaline and it goes faster to the small intestine and is absorbed faster.
-Some drugs are poorly absorbed from any part of the G.I tract because even in their
nonionized state they have low lipid-solubility.
-Other major factors in absorption of drugs in the G.I tract include the concentration of
the drugs, the rate of movement of the contents through the tract, G.I blood flow,
digestive secretion and G.I contents.
-The effect of the movement of the contents through the G.I tract would depend on where
the drug is best absorbed; eg. Rapid emptying of the stomach would decrease the rate of
absorption of weak acids, like aspirin.
-Rapid movement through the intestine could decrease the amount of drug absorbed
because of a shorter duration of contact (or because the drugs is a weak acid)
-Slow movement through the intestine would generally favour drug absorption, due to
better contact with the absorbing cells lining the intestine.
-Because intestinal blood carries drugs away from the intestine and maintain a
concentration gradient conducive to drug absorption.
-Digestive secretion can inactivate some drugs or alter intestinal pH and drug ionization
-The content of the G.I tract that is if there is food can bind to the drug or dilute it so that
it is slowly absorbed.
Disadvantages to oral route