Encyclopedia of Health and Medicine

drug interaction 155
action of another in a known and predictable way
for a therapeutic effect. Such effect occurs, for
example, with the combination of codeine (a narcotic
PAIN reliever) and promethazine (Phenergan),
an antiemetic medication (reduces NAUSEA).
Though an effective pain reliever, codeine tends to
cause nausea, but promethazine offsets this effect.
And though promethazine alone has no analgesic
(pain-relieving) effects it does potentiate, or intensify,
the actions of codeine on the CENTRAL NERVOUS
SYSTEM as well as mitigate its tendency to cause
nausea. Other drug interactions can lessen or
intensify the effects of one or more of the involved
drugs in ways that are detrimental, either by causing
adverse actions in the body or preventing the
therapeutic effects of one or any of the drugs. Certain
ANTIBIOTIC MEDICATIONS, for example, diminish
the effectiveness of oral contraceptives (birth control
pills).
It is important for every doctor, dentist,
or other health-care provider who prescribes
a DRUG for an individual to know
all of the drugs, prescription and overthe-counter
products (including herbal
remedies and natural products) that the
person is taking.
Most drug interactions occur as the result of a
family of enzymes responsible for drug
METABOLISM. These enzymes, called CYTOCHROME
P450 (CYP450) ENZYMES, are abundant in the SMALL
INTESTINE and the LIVER. CYP450 enzymes in the
small intestine initiate the process of metabolism
to allow molecules of the drug’s active ingredient
to pass into the BLOOD circulation. The blood carries
the molecules to the liver, where the CYP450
enzymes there complete metabolism. There are
numerous subtypes of CYP450 enzymes, each
responsible for specific metabolic activity for certain
drugs. Some drugs work by inducing and others
by inhibiting particular CYP450 enzyme
subtypes, which in turn affects the metabolism of
other drugs. Other drug interactions may occur
when the chemicals the drugs contain interact in
some fashion. Iron and calcium in foods, vitamin
supplements, and ANTACIDS bind with some antibiotics
in the STOMACH, for example, preventing the
antibiotic from becoming absorbed and entering
the blood circulation.
The potential for drug interaction is extensive.
The more medications a person takes, the higher
the risk for drug interaction. A useful safeguard is
to ask the pharmacist when picking up a prescription
what other drugs and foods might interact
with it. Even when foods do not directly interact
with drugs, they may affect the drug’s absorption
into the body.
See also ADVERSE DRUG REACTION; ALCOHOL INTER-
ACTIONS WITH MEDICATIONS; ANTIEMETIC MEDICATIONS;
CONTRACEPTION; ILLICIT DRUG ABUSE; OVERDOSE; PRE-
SCRIPTION DRUG ABUSE.
COMMON DRUG/DRUG AND DRUG/FOOD INTERACTIONS
This Drug In Combination with This Drug or Food Consequence of Interaction
anticoagulant medications aspirin further decreases clotting response of the BLOOD,
(heparin, warfarin) GINGKO BILOBA raising risk for bleeding
antiplatelet medications
(cilostazol, clopidogrel, large quantities of spinach increases ability of blood to clot, diminishing
dipyridamole, ticlopidine) vitamin supplement containing VITAMIN K effectiveness of ANTICOAGULANT THERAPY
ANTIFUNGAL MEDICATIONS ALCOHOL of any kind (including in increases the risk for liver failure
(fluconazole, griseofulvin, medications such as cold and flu products)
itraconazole, ketoconazole)