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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