Adverse Drug Effects II

Increased Sensitivity . If certain
body functions develop hyperreactivity,
unwanted effects can occur even at normal
dose levels. Increased sensitivity of
the respiratory center to morphine is
found in patients with chronic lung disease,
in neonates, or during concurrent
exposure to other respiratory depressant
agents. The DRC is shifted to the left
and a smaller dose of morphine is sufficient
to paralyze respiration. Genetic
anomalies of metabolism may also lead
to hypersensitivity. Thus, several drugs
(aspirin, antimalarials, etc.) can provoke
premature breakdown of red blood cells
(hemolysis) in subjects with a glucose-
6-phosphate dehydrogenase deficiency.
The discipline of pharmacogenetics deals
with the importance of the genotype for
reactions to drugs.
The above forms of hypersensitivity
must be distinguished from allergies involving
the immune system .
Lack of selectivity . Despite appropriate
dosing and normal sensitivity,
undesired effects can occur because the
drug does not specifically act on the targeted
(diseased) tissue or organ. For instance,
the anticholinergic, atropine, is
bound only to acetylcholine receptors of
the muscarinic type; however, these are
present in many different organs.
Moreover, the neuroleptic, chlorpromazine,
formerly used as a neuroleptic,
is able to interact with several
different receptor types. Thus, its action
is neither organ-specific nor receptorspecific.
The consequences of lack of selectivity
can often be avoided if the drug
does not require the blood route to
reach the target organ, but is, instead,
applied locally, as in the administration
of parasympatholytics in the form of eye
drops or in an aerosol for inhalation.
With every drug use, unwanted effects
must be taken into account. Before
prescribing a drug, the physician should
therefore assess the risk: benefit ratio.