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 EFFICACYand safety profiles. Therapeuti-
cally equivalent drugs may have superficial differ-
ences 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 BIOAVAILABILITYand efficacy.
The FDA has adopted a BIOEQUIVALENCEstandard
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
LEVELin 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 (thera-
peutically equivalent) or “B” drugs (not therapeu-
tically 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 exam-
ple 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 ingredi-
ents.
See also; GENERIC DRUG; INNOVATOR DRUG.

therapeutic level The amount of a DRUGin the
BLOODcirculation that is necessary to achieve and
sustain the desired effect for treatment, which is

usually a steady state with little variation between

the drug’s PEAK LEVELand 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 concen-

tration 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 meas-

ure 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 P 450 (CYP 450 ) ENZYMES;

DOSAGE; EFFICACY; THERAPEUTIC WINDOW.

therapeutic window The DOSAGE range within

which most of a DRUG’s likely population will

experience the expected EFFICACYand 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 start-

ing dosage.

See also PEAK LEVEL; THERAPEUTIC LEVEL; TROUGH

LEVEL.

trough level The amount of a DRUGin the BLOOD

circulation at the drug’s lowest therapeutic con-

centration. Generally the trough level occurs

immediately before the person is due to take the

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