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P A R T 1
Introduction to nursing pharmacology
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During distribution:
One drug competes for the
protein binding site of another drug, so the second
drug cannot be transported to the responsive
tissue. For example, aspirin competes with the drug
methotrexate for protein-binding sites. Because aspirin
is more competitive for the sites, the methotrexate
is bumped off, resulting in increased release of
methotrexate and increased toxicity to the tissues.
•
During biotransformation:
One drug stimulates
or blocks the metabolism of the other drug. For
example, warfarin, an oral anticoagulant, is
biotransformed more quickly if it is taken at the
same time as barbiturates, rifampicin or many other
drugs. Because the warfarin is biotransformed to
an inactive state more quickly, higher doses will be
needed to achieve the desired effect. People who use
St John’s wort may experience altered effectiveness of
several drugs that are affected by that herb’s effects
on the liver. Cyclosporin, digoxin, theophylline, oral
contraceptives, anticancer drugs, drugs used to treat
HIV and antidepressants are all reported to have
serious interactions with St John’s wort.
•
During excretion:
One drug competes for excretion
with the other drug, leading to accumulation and
toxic effects of one of the drugs. For example, digoxin
and quinidine are both excreted from the same sites
in the kidney. If they are given together, the quinidine
is more competitive for these sites and is excreted,
resulting in increased serum levels of digoxin, which
cannot be excreted.
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At the site of action:
One drug may be an
antagonist of the other drug or may cause effects
that oppose those of the other drug, leading to no
therapeutic effect. This is seen, for example, when
an antihypertensive drug is taken with an antiallergy
drug that increases blood pressure. The effects on
blood pressure are negated and there is a loss of the
antihypertensive effectiveness of the drug. If a person
is taking antidiabetic medication and also takes the
herb ginseng, which lowers blood glucose levels, they
may experience episodes of hypoglycaemia and loss of
blood glucose control.
Whenever two or more drugs are being given
together, first consult a drug guide for a listing of clin
ically significant drug–drug interactions. Sometimes
problems can be avoided by staggering the administra
tion of the drugs or adjusting their doses.
Drug–food interactions
For the most part, a drug–food interaction occurs
when the drug and the food are in direct contact in the
stomach. Some foods increase acid production, speeding
the breakdown of the drug molecule and preventing
absorption and distribution of the drug. Some foods
chemically react with certain drugs and prevent their
absorption into the body. The antibiotic tetracycline
cannot be taken with iron products for this reason.
Tetracycline also binds with calcium to some extent and
should not be taken with foods or other drugs contain
ing calcium. Grapefruit juice has been found to affect
liver enzyme systems for up to 48 hours after it has been
ingested. This can result in increased or decreased serum
levels of certain drugs. Many drugs come with the
warning that they should not be combined with grape
fruit juice. This drug–food interaction does not take
place in the stomach, so the grapefruit juice needs to be
avoided the entire time the drug is being used, not just
while the drug is in the stomach.
In most cases, oral drugs are best taken on an empty
stomach. If the person cannot tolerate the drug on an
empty stomach, the food selected for ingestion with
the drug should be something that is known not to
interact with it. Drug monographs usually list important
drug–food interactions and give guidelines for avoiding
problems and optimising the drug’s therapeutic effects.
Drug–laboratory test interactions
As explained previously, the body works through a
series of chemical reactions. Because of this, adminis
tration of a particular drug may alter results of clinical
tests that are done as part of a diagnostic study. This
drug–laboratory test interaction is caused by the drug
being given and not necessarily by a change in the
body’s responses or actions. Keep these interactions in
mind when evaluating a person’s diagnostic tests. If one
test result is altered and does not fit in with the clinical
picture or other test results, consider the possibility
of a drug–laboratory test interference. For example,
dalteparin (
Fragmin
), a low-molecular-weight heparin
used to prevent deep vein thrombosis after abdominal
surgery, may cause increased levels of the liver enzymes
aspartate aminotransferase (AST) and alanine ami
notransferase (ALT) with no injury to liver cells or
hepatitis.
OPTIMAL THERAPEUTIC EFFECT
As overwhelming as all of this information may seem,
most people can follow a drug regimen to achieve
optimal therapeutic effects without serious adverse
effects. Avoiding problems is the best way to treat
adverse or ineffective drug effects. One should incor
porate basic history and physical assessment factors
Safe medication administration
Always check the monograph of any drug that is being given
to monitor for clinically important drug–drug, drug–alternative
therapy or drug–food interactions.
