apeutic will depend on how fast the drug is absorbed. How long the drug will be
effective and how much drug is needed depends on the route of administration,
the dose of the drug, and the dosage form (tablet, capsule, or liquid).
The absorption rate of a drug is influenced by a number of factors that
might increase or decrease the rate, This is similar to how more gasoline is
used to drive at faster speeds. Absorption is affected by many factors that
include pain, stress, hunger, fasting, food, and pH. Hot, solid, fatty foods can
slow absorption such as eating a Big Mac before taking medication. Even
exercise—which is usually good for the body—affects absorption of a drug.
During exercise, circulation to the stomach is diverted to other areas of the
body and drug absorption is decreased.
Circulation
Blood flow to the site of administration of the drug will help increase the rate of
absorption. An area that has a lot of blood vessels and good circulation will help
absorb the drug quickly and circulate it to the intended site. When a patient is in
shock and has a low blood pressure due to decreased circulation (blood flow)
drugs may not be absorbed very quickly.
Route of Administration
The rate at which drug particles are absorbed is determined by the amount of
blood vessels there are in the area where the drug is administered. Drug particles
are nearly instantaneously absorbed if the drug is injected intravenously (IV). A
slower absorption rate occurs if the drug is administered intramuscularly (IM).
The IM rate is dependent on the amount of blood vessels there are at the site of
the injection. For example, a drug is absorbed faster in the deltoid (arm) muscle
than in the gluteal (butt) muscle because there are more blood vessels in the del-
toid muscle. Drugs injected in subcutaneous (SC) tissue are absorbed slower
than those injected via IM injections because there are fewer blood vessels in
subcutaneous tissues than in muscles.
Solubility
Drug particles dissolve in either lipid (fat) or water. Lipid-soluble drugs are
absorbed more quickly than water-soluble drugs because membranes in the GI
tract are composed of lipids making those membranes a perfect highway for
lipid soluble drugs to move from the GI tract and into the bloodstream.
However, membranes of the GI tract do not directly absorb large water-soluble
CHAPTER 2 Drug Action and Drug Interactions^27