Facts on File Encyclopedia of Health and Medicine

action of another in a known and predictable way
for a therapeutic effect. Such effect occurs, for
example, with the combination of codeine (a nar-
cotic PAINreliever) 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 inten-
sify, the actions of codeine on the CENTRAL NERVOUS
SYSTEMas 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 caus-
ing adverse actions in the body or preventing the
therapeutic effects of one or any of the drugs. Cer-
tain ANTIBIOTIC MEDICATIONS, for example, diminish
the effectiveness of oral contraceptives (birth con-
trol pills).

It is important for every doctor, dentist,

or other health-care provider who pre-

scribes a DRUGfor an individual to know

all of the drugs, prescription and over-

the-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

P 450 (CYP 450 ) ENZYMES, are abundant in the SMALL

INTESTINEand 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 BLOODcirculation. The blood car-

ries 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 cer-

tain drugs. Some drugs work by inducing and oth-

ers 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 ANTACIDSbind with some antibi-

otics 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 prescrip-

tion 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.

drug interaction 155

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 ALCOHOLof any kind (including in increases the risk for liver failure
(fluconazole, griseofulvin, medications such as cold and flu products)
itraconazole, ketoconazole)