
GENERIC NAME: phenytoin
BRAND NAME: Dilantin
DRUG CLASS AND MECHANISM: Phenytoin is an oral and
injectable
anti-seizure medication first synthesized in 1908. Phenytoin was
originally approved by the FDA in 1939.
PRESCRIPTION: yes
GENERIC AVAILABLE: yes, but not recommended.
PREPARATIONS: Tablets (triangular; yellow), 50mg;
capsules
(white with colored band): 30mg (pink band); 100mg (red band).
It also is
available as a suspension (125mg per 5mL).
STORAGE: Tablets, capsules, and suspension should be
kept at
room temperature, 15-30°C (59-86°F).
PRESCRIBED FOR: Although it has been used in many
conditions,
phenytoin's only approved use is as an anti-seizure medication
(anticonvulsant), especially to prevent tonic-clonic (grand
mal) seizures
and complex partial seizures (psychomotor seizures). It may be
used alone
or with phenobarbital or other anticonvulsants.
DOSING: The dosing of phenytoin is very patient-
specific. It may
be given once, twice, or three times daily. Doses are often
adjusted to
find the optimal dose, based on measurement of blood levels.
Taking
phenytoin with food may reduce some of the side effects.
Elderly patients,
debilitated persons, and patients with certain kidney or liver
diseases
may need lower doses. The suspension should not be given at the
same time
as tube feedings.
DRUG INTERACTIONS: There are many potential drug
interactions
with phenytoin. Phenytoin can increase the metabolism
(elimination) of
many drugs, reducing their concentrations in the body. Drugs
that may be
affected include: digoxin, carbamazepine, clonazepam,
corticosteroids
(e.g. prednisone), cyclosporine, disopyramide, doxycycline,
estrogens,
felodipine, levodopa, lidocaine, methadone, mexiletine, oral
contraceptives, paroxetine, quinidine, tacrolimus, theophylline,
phenobarbital, and warfarin. Phenytoin can interact with these
drugs not
only when it is added to therapy but also when it is
discontinued. In the
latter case, the concentration of the other drugs may increase.
Phenytoin's metabolism may be affected by other drugs. Drugs
that can
reduce the amount of phenytoin in the body include rifampin and
phenobarbital. Drugs that increase phenytoin concentrations
include
amiodarone, chloramphenicol, cimetidine, disulfiram,
fluconazole,
fluoxetine, isoniazid (INH), omeprazole, and paroxetine. Thus,
measuring
levels of phenytoin in the blood may be necessary when patients
begin or
discontinue other medications.
The oral absorption of phenytoin can be reduced by any of the
following:
antacids containing magnesium, calcium carbonate, or aluminum;
calcium
salts; or enteral feeding products (tube feedings). Separating
the
administration of phenytoin and enteral feeding products,
antacids, or
calcium salts by at least 2 hours will help avoid this
interaction.
PREGNANCY: There appears to an increased risk of
malformations
and birth defects in women taking phenytoin. Thus phenytoin
should be used
in pregnancy only if the physician feels that the potential
benefit
outweighs the risk.
NURSING MOTHERS: Phenytoin
is secreted into breast
milk. Nursing
is not recommended for persons taking phenytoin.
SIDE EFFECTS: Many varied adverse effects can occur
during
phenytoin therapy including dizziness, drowsiness, difficulty
focusing
(vision), unsteady gate, tiredness, abnormal involuntary
movements,
nausea, vomiting, constipation, abdominal pain, and loss of
appetite.
Children and young adults can develop overgrowth of the gums
during
long-term therapy which requires regular treatment by a
dentist. Good oral
hygiene and gum massage may reduce the risk. Rashes can occur
in between 1
in 20 and 1 in 10 persons; some may be severe. Additionally,
darkening
coloration of the skin may develop (more commonly in women).
Phenytoin can
produce unusual growth of hair in some patients. This reaction
most
commonly affects the arms and legs but can also affect the
trunk and face;
it may be irreversible.
Various lymph node reactions have been reported with phenytoin
therapy.
Lymph nodes may swell up, sometimes painfully. Phenytoin cause
serum
glucose to rise. Thus, blood sugar should be monitored closely
when
phenytoin is administered to patients with diabetes. Phenytoin
can
potentially injure the liver although this is an uncommon
occurrence.
Phenytoin can cause the platelet or white blood cell counts to
drop,
increasing the risk of bleeding or infection, respectively.
Phenytoin also
can cause anemia. Because it interferes with vitamin D
metabolism,
phenytoin can cause weakening of the bones (osteomalacia).
Phenytoin can
cause sexual dysfunction including decreased libido, impotence,
and
priapism (painful, prolonged erections).
Last Editorial Review: 12/31/1997
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