Phenytoin is used to control certain types of convulsions, or seizures. It is not clear exactly how phenytoin works to control convulsions, but it appears to prevent the spread of seizure activity in the brain. Phenytoin may also be used to treat other conditions as determined by your physician.

Phenytoin Facts
Brand names (manufacturers):
Dilantin (Parke-Davis); Dilantin Infatab (Parke-Davis); Dilantin Kapseals (Parke-Davis); phenytoin (various manufacturers)

Type of drug: Anticonvulsant

Ingredient: Phenytoin

Dosage forms: Capsules (30 mg and 100 mg); chewable tablets (50 mg); oral suspension (125 mg per 5-mL spoonful, with less than 0.6% alcohol)

Storage: Store all forms of this drug at room temperature in tightly closed, light-resistant containers. This medication should not be refrigerated and should never be frozen.

Treatment

To avoid stomach irritation, take phenytoin with food or with a full glass of water or milk (unless your doctor directs you to do otherwise). This medication should be taken at the same time in relation to meals to make sure that the drug is absorbed consistently.

Chew the tablet form of this medication before swallowing.

Shake the suspension well just before measuring each dose to distribute the ingredients evenly and equalize the doses. The contents tend to settle on the bottom of the bottle. Measure each dose carefully with a specially designed 5-mL measuring spoon. An ordinary kitchen teaspoon is not accurate enough.

Phenytoin works best when the level of medicine in your bloodstream is kept constant. It is best to take the doses at evenly spaced intervals day and night. For example, if you are taking three doses a day, the doses should be spaced eight hours apart.

If you miss a dose of phenytoin, take the missed dose as soon as possible. However, if it is almost time for the next dose, do not take the missed dose at all; just return to your regular dosing schedule. Do not double the next dose. If you miss two or more doses in a row, contact your doctor.

Side Effects

Minor: Constipation, drowsiness (mild), headache, insomnia, nausea, or vomiting. These side effects should begin to disappear as your body gets accustomed to the medication.

To relieve constipation, increase the amount of fiber in your diet (fresh fruits and vegetables, salads, bran, and whole-grain breads), exercise, and drink more water (unless your doctor directs you to do otherwise).

Major: Tell your doctor about any side effects that are persistent or particularly bothersome. IT IS ESPECIALLY IMPORTANT TO TELL YOUR DOCTOR about blurred vision, change in facial features, change in the color of your urine, chest pain, confusion, dizziness, gum enlargement, increased hair growth, joint pain, muscle twitching, nervousness, numbness, rash, slurred speech, sore throat, swollen glands, uncoordinated movements, unusual bleeding or bruising, or yellowing of the eyes or skin.

Interactions

Phenytoin can be expected to interact with a number of other types of medications:
  • The effectiveness of phenytoin can be decreased by concurrent use of alcohol, barbiturates, tricyclic antidepressants, phenothiazines, bupropion, antipsychotics (such as clozipine), and haloperidol. Concurrent use of these medications with phenytoin may increase the risk of seizures.

  • Phenytoin can decrease the effectiveness of amiodarone, calcifediol, warfarin, quinidine, disopyramide, dexamethasone, doxycycline, lamotrigine, levodopa, and theophylline.

  • The active blood levels and also the side effects of phenytoin can be increased by other anticonvulsants, chloramphenicol, cimetidine, disulfiram, isoniazid, aspirin, fluoxetine, metronidazole, amiodarone, trimethoprim, sulfonamide antibiotics, tolbutamide, benzodiazepines, tricyclic antidepressants, phenothiazines, estrogens, warfarin, omeprazole, ticlopidine, methylphenidate, fluconazole, itraconazole, or ketoconazole.

  • Valproic acid can either increase or decrease the effects of phenytoin.

  • The dosage of oral antidiabetic medications or insulin may need to be adjusted when phenytoin is started.

  • Phenytoin may decrease the absorption of furosemide from the gastrointestinal tract, decreasing furosemide's effectiveness.

  • Antacids, calcium supplements, sucralfate, didanosine, and medicines for diarrhea may decrease the gastrointestinal absorption and effectiveness of phenytoin. Do not take phenytoin within two to three hours of taking an antacid or antidiarrheal.

  • Phenytoin may decrease the effectiveness of oral contraceptives (birth control pills). An alternative method of birth control may be needed. Check with your doctor.

  • Concurrent use of folic acid, leucovorin, rifampin, paroxetine, or enteral feeding (such as drinking Ensure) may decrease the effectiveness of phenytoin by lowering the amount of medication in the bloodstream.
Before starting to take phenytoin, BE SURE TO TELL YOUR DOCTOR about any medications you are currently taking, especially any of those listed above.

Warnings
  • Tell your doctor about unusual or allergic reactions you have had to any medications, especially to phenytoin, ethotoin, or mephenytoin.

  • Before starting to take phenytoin, be sure to tell your doctor if you now have or ever had blood disorders, diabetes mellitus, or liver disease.

  • If phenytoin makes you dizzy or drowsy, do not take part in any activity that requires alertness, such as driving a car or operating potentially dangerous machinery. Children should be careful while playing.

  • Before surgery or other medical or dental treatment, tell your doctor or dentist you are taking phenytoin.

  • Do not stop taking phenytoin unless you first consult your doctor. If this drug is stopped abruptly, you may experience uncontrollable seizures. Your doctor may want to reduce your dosage gradually. Be sure you have enough on hand for holidays and vacations.

  • Although several generic versions of phenytoin are available, you should not switch from one brand to another without your doctor's prior assessment and approval. If you have your medication refilled and it looks different, be sure to consult with your pharmacist.

  • Therapy with phenytoin may cause your gums to enlarge enough to cover your teeth. This can be minimized, at least partially, by frequent brushing and massaging of the gums with the rubber tip of a good toothbrush.

  • Be sure to tell your doctor if you are pregnant. Birth defects have been reported more often in infants whose mothers have seizure disorders. It is unclear if the increased risk of birth defects is associated with the seizure disorders or with anticonvulsant medications such as phenytoin that are used to treat them. Discuss this with your doctor. Also tell your doctor if you are breast-feeding an infant. Phenytoin passes into breast milk.