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Drug therapy in the older adult
If you are providing drug therapy for older
adults, you will want to understand physi-
ological and pharmacokinetic changes that
may alter appropriate drug dosage or cause
common adverse reactions or compliance
problems in these people.
Physiological changes
As a person ages, gradual physiological
changes occur. Some of these age-related
changesmay alter the therapeutic and toxic
effects of medications.
Body composition
Proportions of fat, lean tissue and water in
the body change with age.Total body mass
and lean body mass tend to decrease; the
proportion of body fat tends to increase.
Varying from person to person, these
changes in body composition affect the
relationship between a drug’s concen-
tration and distribution in the body. For
example, a water-soluble drug, such as gen-
tamicin, is not distributed to fat. Because
there is relatively less lean tissue in an older
person, more of a drug remains in the blood.
GI function
In older adults, decreases in gastric acid
secretion and GI motility slow the emptying
of stomach contents and the movement of
intestinal contents through the entire tract.
Furthermore, research suggests that older
adults may have more difficulty absorbing
medications.This is a particularly significant
problemwith drugs having a narrow thera-
peutic range, such as digoxin, in which any
change in absorption can be crucial.
Hepatic function
The ability of the liver tometabolise certain
drugs decreases with age because of dimin-
ished blood flow to the liver—a result of the
age-related decrease in cardiac output and
diminished activity of certain liver enzymes.
When an older person takes certain sleep
medications, such as nitrazepam, reduced
liver metabolisation of the drug may cause
a hangover effect the next morning.
Decreased hepatic function may lead to
the following:
• more intense drug effects due to higher
blood levels
• longer-lasting drug effects due to
prolonged blood concentrations
• greater incidence of drug toxicity.
Renal function
Although an older person’s renal function
is usually sufficient to eliminate excess
body fluid and waste, the ability to elimi-
nate some medications may be reduced by
50%or more. Manymedications commonly
used by older adults, such as digoxin, are
excreted primarily through the kidneys. If
renal excretion of the drug decreases, high
blood concentrations may result. Digoxin
toxicity, therefore, is relatively common
in older adults who are not receiving a
reduced digoxin dosage to accommodate
decreased renal function.
Drug dosages can be modified to com-
pensate for age-related decreases in renal
function. Aided by laboratory tests, such
as urea and serum creatinine, prescribers
may adjust medication dosages so that the
person receives the expected therapeutic
benefits without the risk of toxicity. Since
creatinine is a by-product of musclemetab-
olism, in older individuals serum creatinine
may remain in the normal range despite
a falling glomerular filtration rate because
of low muscle mass. Thus, elderly people
should be considered as having impaired
renal function and their drug dosage
should generally be substantially lower
than for younger people. It is common to
start with about 50% of the adult dose.
