phenytoin (Dilantin)

Phenytoin is an oral and injectable anti-seizure medication first synthesized in 1908.

Phenytoin is an anti-seizure medication (anticonvulsant) used for preventing or treating generalized tonic-clonic (grand mal) seizures, complex partial seizures (psychomotor seizures), and seizures occurring during or after neurosurgery. It may be used alone or with phenobarbital or other anticonvulsants.

Yes

Yes

Many 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. R

ashes 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, sometimes painfully.

Phenytoin causes serum glucose (sugar) 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).

Other important side effects caused by phenytoin include sexual dysfunction, such as:

• decreased libido,
• impotence, and
• priapism (painful, prolonged erections).

Antiepileptic medications have been associated with an increased risk of suicidal thinking and behavior. Anyone considering the use of antiepileptic drugs must balance this risk of suicide with the clinical need for the antiepileptic drug. Patients who begin antiepileptic therapy should be closely observed for clinical worsening, suicidal thoughts or unusual changes in behavior.

The dosing of phenytoin is patient specific. It may be given once, twice, 3, or 4 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 since tube feedings bind to phenytoin and reduce its absorption. The recommended adult dose is 100 mg two to four times daily. Some patients may require 200 mg three times daily. Patients stabilized on 100 mg three times daily may receive 300 mg once daily of the extended release capsules.

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 (Lanoxin),
• carbamazepine (Tegretol, Tegretol XR, Equetro, Carbatrol),
• clonazepam (Klonopin),
• corticosteroids (for example, prednisone),
• cyclosporine,
• disopyramide,
• doxycycline,
• estrogens,
• oral contraceptives,
• paroxetine (Paxil, Paxil CR, Pexeva),
• quinidine,
• tacrolimus (Prograf),
• theophylline,
• phenobarbital,
• valproic acid (Depakote, Depakote ER, Depakene, Depacon, Stavzor), and
• warfarin (Coumadin).

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. Sometimes the effect may be unpredictable. Phenytoin's metabolism may be affected by other drugs. Drugs that increase phenytoin blood levels and toxicity include:

• acute alcohol intake,
• amiodarone (Coradone),
• chlordiazepoxide,
• cimetidine (Tagamet),
• diazepam (Valium, Diastat),
• dicumarol,
• disulfiram,
• estrogens,
• ethosuximide,
• fluoxetine (Prozac),
• fluorouracil,
• fluvoxamine (Luvox),
• isoniazid,
•  methylphenidate (Concerta),
• omeprazole (Prilosec),
• sertraline (Zoloft),
• tolbutamide, and
• trazodone (Desyrel).

Drugs that may decrease phenytoin levels and reduce effectiveness include carbamazepine, chronic alcohol abuse, reserpine, and sucralfate (Carafate).

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.

There is 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.

Phenytoin is secreted into breast milk. Breastfeeding is not recommended for persons taking phenytoin.

What preparations of Dilantin are available?

Capsules (extended or immediate release): 30, 100, 200, and 300 mg; Chewable Tablet: 50 mg; Suspension: 125 mg/5 mL; Injection: 50 mg/mL.

How should I keep Dilantin stored?

Capsules, tablets, and suspension should be kept at room temperature, 68 F - 77 F (20 C - 25 C).

How does Dilantin work?

The exact mechanism of action is not known, Phenytoin may work by reducing the sensitivity of nerves in the brain to excessive stimulation and reducing transmission of impulses between nerves.

When was Dilantin approved by the FDA?

Phenytoin was originally approved by the FDA in 1939.