P–S

peak level The maximum concentration of a
DRUGin the BLOODcirculation. The peak level cor-
responds in part to the drug’s route of administra-
tion, chemical composition, and rate of
METABOLISM. A drug’s peak level may occur within
minutes to several hours of taking or receiving it.
Injectable drugs enter the blood circulation rap-
idly; oral medications (taken by MOUTH) take
longer to reach the blood as they must first go
through digestion. Foods and liquids also con-
sumed affect the rate of digestion and absorption,
as do other factors such as the person’s activity
level, age, body weight, and any health condi-
tions.
A drug’s peak level establishes the upper limit
of the drug’s therapeutic range. For most drugs it
is not necessary for the doctor to determine peak
level and TROUGH LEVEL(lowest concentration) as
the drug’s informational literature provides the
expected levels. Improvement in the person’s
symptoms or condition is clinical evidence that the
drug DOSAGE is therapeutically appropriate. The
doctor may more closely monitor blood concen-
trations for NARROW THERAPEUTIC INDEX(NTI) drugs,
for which the peak and trough levels are critical.
Because the goal of most medication therapy is to
achieve a fairly constant level of the drug in the
blood circulation, peak and trough levels are pri-
marily significant at the onset of treatment.
See also HALF-LIFE; ROUTES OF ADMINISTRATION;
THERAPEUTIC LEVEL.

pharmacodynamics The actions of drugs within
the body. Drugs enter and act within the body by
binding with cell receptors, specialized fragments
of proteins that instruct the cell to take or not take
specific actions. This binding process, called selec-
tivity, limits and directs the effects of drugs. Pro-

teins are the basic components of the body’s bio-

chemical messengers, hormones and neurotrans-

mitters. The interactions they initiate are often

intricate cascades that influence numerous bio-

chemical processes (such as ion passage for cell

communication) as well as DNAencoding and tran-

scription (cell function and replication). Numer-

ous factors influence the unfolding of these

cascades, from a person’s general health status and

existing health conditions to other drugs the per-

son is taking. For example, INSULIN RESISTANCEand

DIABETESaffect the energy accessible to cells to

carry out the functions of cellular METABOLISM,

altering the processes and outcomes of receptor

binding. Through different mechanisms HYPOTHY-

ROIDISMslows and HYPERTHYROIDISMaccelerates cel-

lular metabolism, also affecting receptor binding.

Pharmacodynamics gives doctors and pharmacists

the ability to assess how individuals may react to

specific drugs, depending on their unique health

profiles.

See also ALCOHOL INTERACTIONS WITH MEDICATIONS;

BIOAVAILABILITY; CYTOCHROME P 450 (CYP 450 ) ENZYMES;

HORMONE; NEURON; NEUROTRANSMITTER; PHARMACOKI-

NETICS.

pharmacokinetics The timing of a DRUG’s

absorption, METABOLISM, action, and excretion.

Pharmacokinetics is an element of DOSAGEdeter-

mination and the EFFICACYof a drug. Many vari-

ables unique to an individual influence the rate of

a drug’s entry into, stay within, and passage from

the body. Mathematical calculations that integrate

the drug’s characteristics (such as form and

strength) with an individual’s health circum-

stances allow doctors and pharmacists to tailor

medication therapy regimens specific to the indi-

vidual’s needs.

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