Encyclopedia of Health and Medicine

T
therapeutic equivalence In pharmacology, drugs
that have the same active ingredients in the same
forms and have the same actions within the body.
The US Food and Drug Administration (FDA),
which oversees DRUG approval in the United
States, refers to such drugs as bioequivalent with
matching EFFICACY and safety profiles. Therapeutically
equivalent drugs may have superficial differences
such as in appearance (shape or color) and
the inactive ingredients that serve as the vehicle to
contain the active ingredient. However, they must
have the same BIOAVAILABILITY and efficacy.
The FDA has adopted a BIOEQUIVALENCE standard
based on a statistical methodology in which the
time it takes for each drug to reach its maximum
concentration in the BLOOD circulation and the
amount of time the drug remains at a THERAPEUTIC
LEVEL in the blood circulation differ by no more
than 20 percent. In its official listing of approved
drugs, The Orange Book, the FDA identifies all
drugs with alternate products as “A” drugs (therapeutically
equivalent) or “B” drugs (not therapeutically
equivalent).
Health-care providers other than pharmacists
sometimes use the term therapeutic equivalence
in the context of different drugs within the same
classification that have similar effects—for example
the drugs fluoxetine and sertraline, both of
which are selective serotonin reuptake inhibitors
(SSRIs) to treat DEPRESSION. Though these drugs act
in similar ways to achieve a similar therapeutic
effect, they do not have the same active ingredients.
See also; GENERIC DRUG; INNOVATOR DRUG.
therapeutic level The amount of a DRUG in the
BLOOD circulation that is necessary to achieve and
sustain the desired effect for treatment, which is
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usually a steady state with little variation between
the drug’s PEAK LEVEL and TROUGH LEVEL. Doctors
calculate dosages to achieve a therapeutic level,
factoring the person’s age, body weight, and other
medications with which interactions are possible.
For most drugs, blood drawn at any time provides
the needed information about the drug’s concentration
in the blood. At the onset of medication
therapy or when taking a drug that has a NARROW
THERAPEUTIC INDEX (NTI), blood tests taken to measure
both peak and trough levels may provide
more useful information to assess whether the
drug is at therapeutic level. It often is valuable to
tell the doctor or the lab the time of the last DOSE
of the medication, which may help to determine
whether doses are spaced appropriately.
See also CYTOCHROME P450 (CYP450) ENZYMES;
DOSAGE; EFFICACY; THERAPEUTIC WINDOW.
therapeutic window The DOSAGE range within
which most of a DRUG’s likely population will
experience the expected EFFICACY and therapeutic
value of the drug. The therapeutic window is
important to doctors when they calculate dosages,
providing a clinically valid starting point for most
people. Individual characteristics such as other
health conditions, other medications being taken,
body weight, and activity level help the doctor
determine where within the therapeutic window
is the most appropriate point to choose the starting
dosage.
See also PEAK LEVEL; THERAPEUTIC LEVEL; TROUGH
LEVEL.
trough level The amount of a DRUG in the BLOOD
circulation at the drug’s lowest therapeutic concentration.
Generally the trough level occurs
immediately before the person is due to take the