Tachyphylaxis.The patient builds a tolerance to the drug due to the frequency
in which the drug is administered. This occurs with narcotics, barbiturates, lax-
atives, and psychotropic agents. The patient may eventually need more of the
drug to reach the desired effect.
Placebo effect.The patient receives a psychological benefit from receiving
a compound that has no pharmaceutical response. A third of patients taking a
placebo experience the placebo effect.
Pharmacogenetic effect.A drug varies from a predicted response because of
the influence of a patient’s genetic factors. Genetic factors can alter the metab-
olism of the drug and results in an enhanced or diminished pharmaceutical
response.
Allergic reactions.If the patient was previously sensitized to the drug, a drug
might trigger the patient’s immunologic mechanism that results in allergic symp-
toms. Antibodies are produced the first time the drug is introduced to the patient
creating a sensitivity to the drug. The next time the drug is given to the patient,
the drug reacts with the antibodies and results in the production of histamine.
Histamine causes allergic symptoms to occur. The patient should not take any
drug that causes the patient to have an allergic reaction.
There are four types of allergic reactions. These are:- Anaphylactic. This is an immediate allergic reaction that can be fatal.
- Cytotoxic reaction. This is an autoimmune response that results in hemo-
lytic anemia, thrombocytopenia, or lupus erythematosus (blood disorders).
In some cases, it takes months for the reaction to dissipate. - Immune complex reaction. This is referred to as serum sickness and results
in angioedema, arthralgia (sore joints), fever, swollen lymph nodes, and
splenomegaly (large spleen). The immune complex reaction can appear up
to three weeks after the drug is administered. - Cell mediated. This is an inflammatory skin reaction that is also known as
delayed hypersensitivity.
Summary
A drug has a physiochemical action with the physiological process of the body
resulting in a pharmaceutical response. Drugs replace a missing element such as
a hormone, interrupts a physiological process or stimulates a physiological
process to occur. In addition to a therapeutic effect, drugs may have side effects
that can be desirable or undesirable.(^38) CHAPTER 2 Drug Action and Drug Interactions