body produces) that participate in the METABOLISM
of drugs. The CYP450 enzymes also participate in
lipid (notably cholesterol) and steroid HORMONE
synthesis. Most of the CYP450 enzymes that are
active in DRUGmetabolism are in the LIVERand the
SMALL INTESTINE. The CYP450 enzymes function as
catalysts to facilitate the processes by which the
drug transforms from its initial chemical structure
to the biochemical forms that have action in the
body. Each of the CYP450 subtypes, also called
isoforms or isoenzymes, metabolizes certain drugs
or groups of drugs.
Hormones, antibodies, and foods affect the
activity of CYP450 enzymes. Interactions among
them may block or enhance a drug’s activities;
these effects may be beneficial or harmful. Some
drug treatment regimens for complex conditions
such as HIV/AIDSwork by manipulating CYP450
enzyme activity to take advantage of beneficial
interactions. Harmful interactions may manifest as
adverse drug reactions such as toxicity or unpleas-
ant side effects.
Individuals may express CYP450 activity differ-
ently—that is, known variation exists among indi-
viduals in the ways CYP450 enzymes function.
These variations in CYP450 expression factor into
individual responses to medications, at least par-
tially accounting for why one drug may be more
or less effective than another drug in the same
drug family for a particular individual.
See also ALCOHOL INTERACTIONS WITH MEDICATIONS;
ANTIBODY; PHARMACODYNAMICS; PHARMACOKINETICS.
dosage The therapeutic course of a DRUG,
encompassing the drug’s DOSE(amount of the drug
taken), the frequency of the doses, the health con-
dition and status of the person (including age and
gender), and the total length of time the drug the
person needs to take the drug. For many drugs
there are standard dosages that are applicable to
most people. The doctor or pharmacist calculates
dosages for people who fall outside the standard
range, and for NARROW THERAPEUTIC INDEX (NTI)
drugs (drugs for which the margin between thera-
peutic and toxic is very close). People who may
fall outside the standard range of dosage for many
drugs are the very young, the very old, those who
are extremely underweight, those who are
extremely overweight, those who have multiple
health conditions, and those who take numerous
medications.
See also AGING, EFFECTS ON DRUG METABOLISM AND
DRUG RESPONSE; PEAK LEVEL; THERAPEUTIC LEVEL;
TROUGH LEVEL.dose The amount of a DRUGa person takes or
receives at a single time. A dose falls within a rec-
ommended therapeutic range for the drug, the
person’s condition, and the person’s personal
health circumstances (including age and gender).
An excess of this amount is an OVERDOSE, which
can have serious and even fatal consequences.
See also AGING, EFFECTS ON DRUG METABOLISM AND
DRUG RESPONSE; DOSAGE; PEAK LEVEL; THERAPEUTIC
LEVEL; THERAPEUTIC WINDOW; TROUGH LEVEL.drug Any product that, when it enters the body,
changes the function of the body in some way.
Drugs such as ANTIBIOTIC MEDICATIONS work by
killing BACTERIAwithin the body, for example, and
antiarrhythmia drugs work by altering the electri-
cal activity of the HEART. As the mainstay of mod-
ern medicine, drugs exert therapeutic actions to
treat numerous health conditions.
See also ADVERSE DRUG REACTION; ALCOHOL; DRUG
INTERACTION; INVESTIGATIONAL NEW DRUG(IND); OFF-
LABEL USE.drug interaction An effect or action that occurs
in the body as a consequence of taking two or
more drugs that does not occur when taking any
one of the drugs alone. Drugs may interact with
each other, OVER-THE-COUNTER (OTC) DRUGS and
products, vitamin and mineral supplements,
MEDICINAL HERBS AND BOTANICALS, and foods. Most
DRUGinteractions are inadvertent, occurring when
a person takes an OTC medication with prescrip-
tion medications, for example, or when a doctor
prescribes a new medication without knowing all
of the other medications a person is taking. The
latter circumstance becomes a particular challenge
when a person must receive urgent care in a
clinic, hospital emergency department, or other
setting in which the provider is someone other
than the person’s regular health-care provider.
Some drug interactions are neutral or even
beneficial, such as when one medication potenti-
ates (increases or enhances) or mitigates the154 Drugs