- Form: tablet, capsule, slow-release, liquid, transdermal patch, suppository,
and inhalation. - Route: PO (mouth), topical, parenteral, and rectal.
- GI: The ability of the mucosa (lining) in the GI tract impacts the ability to
absorb drug particles and the ability to move food through the digestive tract. - Food: Drug particles for some drugs are better absorbed if they are taken
with certain foods, while other foods slow down or block absorption. - Drugs: Some drugs increase or decrease another drug’s absorption when
both drugs are taken together. - Liver metabolism: Liver dysfunction can prevent or delay the metabolism
of a drug. - Concentration: A higher portion of active ingredient in a dose increases the
amount of drug particles that are absorbed. - Cell membrane: Single layer cell membrane, such as those found in the
intestine, increase absorption, while some drugs are absorbed more slowly
in multiple-layers, such as skin. - Surface area: A larger surface area, such as in the small intestine, absorbs
drugs faster than a smaller area such as in the stomach.
DRUG CONCENTRATION
A drug contains an active ingredient, which produces the therapeutic effect, and
other materials that give the drug form and protection. The percent of active
ingredient in a dose is referred to as the drug concentration.
There are generally two levels of concentrations. These are primary loading—
a large concentration that is used to achieve a fast therapeutic effect such as the
first dose of an antibiotic, and maintenance dose—a typical concentration of
the drug that is used to provide an ongoing therapeutic effect such as subsequent
doses of an antibiotic.
DISTRIBUTION
Once absorbed, drug particles are transported in blood plasma. These are
referred to as “free” drugs because they are not bound to any receptor sites. Only
free drugs can cause a pharmacological response. Drugs bind to proteins in
plasma, usually albumin or globulins. These drug–protein complexes decrease
the concentration of free drug in the circulation. This protein–drug molecule is
too large to pass through the membrane of a blood vessel and is not available for
CHAPTER 2 Drug Action and Drug Interactions^29