Pharmacokinetics VIII

Change in Elimination Characteristics
During Drug Therapy
With any drug taken regularly and accumulating
to the desired plasma level, it
is important to consider that conditions
for biotransformation and excretion do
not necessarily remain constant. Elimination
may be hastened due to enzyme
induction (p. 32) or to a change in urinary
pH (p. 40). Consequently, the
steady-state plasma level declines to a
new value corresponding to the new
rate of elimination. The drug effect may
diminish or disappear. Conversely,
when elimination is impaired (e.g., in
progressive renal insufficiency), the
mean plasma level of renally eliminated
drugs rises and may enter a toxic concentration
range.