molecules and a carrier must be used to transport the water-soluble drugs across
the GI membrane and into the bloodstream. This additional step causes water-
soluble drugs to be absorbed more slowly than fat-soluble drugs.pH Level
The pH level of a drug determines how easily drug particles will be absorbed
in the GI tract. Those drugs that are a weak acid—such as aspirin— can pass rap-
idly across the GI tract membrane while weak base drugs—such as an antacid—
are absorbed more slowly than a weak acidic drug. Strong acids and bases destroy
cells and are not absorbed.
The concentration of the drug will also affect the rate of absorption. If a high
concentration of the drug is given, it will tend to be absorbed more rapidly.
Sometimes larger (loading or priming) doses of a drug may be given that will be
more than the body can excrete. When this is done, the drug becomes therapeu-
tic much faster. After the first large dose, small maintenance doses will help keep
the therapeutic effect.
The form (solid, liquid) the drug is given can affect the absorption rate.
Drugs can be processed when they are manufactured to add other ingredients
that will help or hinder absorption.BIOAVAILABILITY
Not all drug particles reach the circulatory system. Some particles are mis-
directed or destroyed during the absorption process. For example, hydrochloric
acid in the stomach destroys some drug particles before it can pass through the
membrane and into the bloodstream.
The percentage of a dose that reaches the blood stream is called the bio-
availabilityof a drug. Typically, between 20% and 40% of drugs that are admin-
istered orally reach the blood stream. This is called the first pass effect and is the
beginning of the metabolism of a drug that is given orally. After a drug is absorbed
in the GI tract it is carried to the liver and metabolism occurs. Sometimes very lit-
tle of the drug remains available for a therapeutic effect after the first pass. Only
drugs administered intravenously have a 100% bioavailability because they are
directly injected into the vein.
Pharmaceutical manufacturers must consider bioavailability when determin-
ing the dose for a drug. For example, the dose for a drug administered PO (orally)
might be 4 times higher than if the same drug is administered intravenously.
There are a number of factors that alter bioavailability. These are:(^28) CHAPTER 2 Drug Action and Drug Interactions