The First Pass Effect
The most common way drugs are administered is orally, by swallowing a pill. The
drug is then absorbed into the GI tract and enters the portal circulation system
where drug particles are transported through the portal vein into the liver where
the drug is metabolized. This is referred to as the first pass effect.
Not all drugs are metabolized in the liver. Some drugs bypass the first pass
effect by sublingual administration (under the tongue) or buccal administra-
tion(between the gums and the cheek) where they are absorbed directly into
the bloodstream from the mouth. These drugs do not enter the stomach where the
hydrochloric acid might destroy drug particles. Other drugs go directly to the liver
through the portal vein and also bypass the stomach. The drug is then metabo-
lized in the liver and much of the drug may be eliminated and not available for
a therapeutic effect. Sometimes this effect is so great that none of the drug is
available for use if given by mouth. The drug must then be given in very high
doses or parenterally (intramuscularly or intravenously) to bypass the liver.
Pharmacodynamics
Pharmacodynamics is a drug’s effect on the physiology of the cell and the mech-
anism that causes the pharmaceutical response. There are two types of effects
that a drug delivers. These are the primary effect and the secondary effect. The
primary effect is the reason for which the drug is administered. The secondary
effect is a side effect that may or may not be desirable.
For example, diphenhydramine (Benadryl) is an antihistamine. Its primary
effect is to treat symptoms of allergies. Its secondary effect is to depress the cen-
tral nervous system causing drowsiness. The secondary effect is desirable if the
patient needs bedrest, but undesirable if the patient is driving a car.
A period of time passes after a drug is administered until the pharmaceutical
response is realized. This is referred to as the drug’s time response. There are
three types of time responses: onset, peak, and duration.
The onset time response is the time for the minimum concentration of drug to
cause the initial pharmaceutical response. Some drugs reach the onset time in
minutes while other drugs take days. The peak time response is when the drug
reaches its highest blood or plasma concentration. Duration is the length of time
that the drug maintains the pharmaceutical response.
CHAPTER 2 Drug Action and Drug Interactions^33