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Some commonly used brand names are:

In the U.S.—

  • Cerebyx 2
  • Dilantin 4
  • Dilantin-125 4
  • Dilantin Infatabs 4
  • Dilantin Kapseals 4
  • Mesantoin 3
  • Peganone 1
  • Phenytek 4

In Canada—

  • Cerebyx 2
  • Dilantin 4
  • Dilantin-30 4
  • Dilantin-125 4
  • Dilantin Infatabs 4

Another commonly used name for phenytoin is diphenylhydantoin .


For quick reference, the following hydantoin anticonvulsants are numbered to match the corresponding brand names.

This information applies to the following medicines:
1. Ethotoin (ETH-oh-toyn)
2. Fosphenytoin (fos-FEN-i-toyn)
3. Mephenytoin (me-FEN-i-toyn)
4. Phenytoin (FEN-i-toyn)
† Not commercially available in Canada
‡ Generic name product may be available in the U.S.


  • Antiarrhythmic—Phenytoin
  • Anticonvulsant—Ethotoin; Fosphenytoin; Mephenytoin; Phenytoin
  • Antineuralgic, trigeminal neuralgia—Phenytoin
  • Skeletal muscle relaxant—Phenytoin


Hydantoin anticonvulsants (hye-DAN-toyn an-tye-kon-VUL-sants) are used most often to control certain convulsions or seizures in the treatment of epilepsy. Phenytoin also may be used for other conditions as determined by your doctor.

In seizure disorders, these medicines act on the central nervous system (CNS) to reduce the number and severity of seizures. Hydantoin anticonvulsants may also produce some unwanted effects. These depend on the patient"s individual condition, the amount of medicine taken, and how long it has been taken. It is important that you know what the side effects are and when to call your doctor if they occur.

Hydantoin anticonvulsants are available only with your doctor"s prescription, in the following dosage forms:

  • Oral
  • Ethotoin
    • Tablets (U.S.)
  • Mephenytoin
    • Tablets (U.S.)
  • Phenytoin
    • Extended capsules (U.S. and Canada)
    • Prompt capsules (U.S.)
    • Oral suspension (U.S. and Canada)
    • Chewable tablets (U.S. and Canada)
  • Parenteral
  • Fosphenytoin
    • Injection (U.S. and Canada)
  • Phenytoin
    • Injection (U.S. and Canada)


Because fosphenytoin is converted to phenytoin in your body, it has the same effects as those listed for phenytoin in the following sections.

Before Using This Medicine

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For hydantoin anticonvulsants, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to any hydantoin anticonvulsant medicine. Also tell your health care professional if you are allergic to any other substance, such as foods, preservatives, or dyes.

Pregnancy—Although most mothers who take medicine for seizure control deliver normal babies, there have been reports of increased birth defects when these medicines were used during pregnancy. It is not definitely known if any of these medicines are the cause of such problems.

Also, pregnancy may cause a change in the way hydantoin anticonvulsants are absorbed in your body. You may have more seizures, even though you are taking your medicine regularly. Your doctor may need to increase the anticonvulsant dose during your pregnancy.

In addition, when taken during pregnancy, this medicine may cause a bleeding problem in the mother during delivery and in the newborn. This may be prevented by giving vitamin K to the mother during delivery, and to the baby immediately after birth.

Breast-feeding—Ethotoin and phenytoin pass into the breast milk in small amounts. It is not known whether mephenytoin passes into breast milk. Be sure you have discussed the risks and benefits of the medicine with your doctor.

Children—Some side effects, especially bleeding, tender, or enlarged gums and enlarged facial features, are more likely to occur in children and young adults. Also, unusual and excessive hair growth may occur, which is more noticeable in young girls. In addition, some children may not do as well in school after using high doses of this medicine for a long time.

Older adults—Some medicines may affect older patients differently than they do younger patients. Overdose is more likely to occur in elderly patients and in patients with liver disease.

Other medicines—Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking or receiving hydantoin anticonvulsants, it is especially important that your health care professional know if you are taking any of the following:

  • Alcohol or
  • Central nervous system (CNS) depressants (medicine that causes drowsiness)—Long-term use of alcohol may decrease the blood levels of hydantoin anticonvulsants, resulting in decreased effects; use of hydantoin anticonvulsants in cases where a large amount of alcohol is consumed may increase the blood levels of the hydantoin, resulting in an increased risk of side effects
  • Amiodarone (e.g., Cordarone)—Use with phenytoin and possibly with other hydantoin anticonvulsants may increase blood levels of the hydantoin, resulting in an increase in serious side effects
  • Antacids or
  • Medicine containing calcium—Use of antacids or calcium supplements may decrease the absorption of phenytoin; doses of antacids and phenytoin or calcium supplements and phenytoin should be taken 2 to 3 hours apart
  • Anticoagulants (blood thinners) or
  • Chloramphenicol (e.g., Chloromycetin) or
  • Cimetidine (e.g., Tagamet) or
  • Disulfiram (e.g., Antabuse) (medicine for alcoholism) or
  • Isoniazid (INH) (e.g., Nydrazid) or
  • Fluconazole (e.g., Diflucan) or
  • Fluoxetine (e.g., Prozac) or
  • Itraconazole (e.g., Sporanox) or
  • Ketoconazole (e.g., Nizoral) or
  • Miconazole (e.g., Monistat) or
  • Phenylbutazone (e.g., Butazolidin) or
  • Sulfonamides (sulfa drugs)—Blood levels of hydantoin anticonvulsants may be increased, increasing the risk of serious side effects; hydantoin anticonvulsants may increase the effects of the anticoagulants at first, but with continued use may decrease the effects of these medicines
  • Corticosteroids (cortisone-like medicines) or
  • Estrogens (female hormones) or
  • Oral contraceptives (birth-control pills) containing estrogens or progestins or
  • Progestin injection contraceptives (e.g., Depo-Provera) or
  • Progestin implant contraceptives (e.g., Norplant)—Hydantoin anticonvulsants may decrease the effects of these medicines; use of hydantoin anticonvulsants with estrogen- or progestin-containing contraceptives may result in breakthrough bleeding and contraceptive failure; additional birth control measures may be needed to decrease the risk of pregnancy
  • Diazoxide (e.g., Proglycem)—Use with hydantoin anticonvulsants may decrease the effects of both medicines; therefore, these medicines should not be taken together
  • Felbamate (e.g., Felbatrol)—Blood levels of hydantoin anticonvulsants may be increased, and blood levels of felbamate may be decreased. Your doctor may need to adjust your dosage
  • Lidocaine—Risk of slow heartbeat may be increased. Other effects of lidocaine may be decreased because hydantoin anticonvulsants may cause it to be removed from the body more quickly
  • Methadone (e.g., Dolophine, Methadose)—Long-term use of phenytoin may bring on withdrawal symptoms in patients being treated for drug dependence
  • Phenacemide (e.g., Phenurone)—Use with hydantoin anticonvulsants may increase the risk of serious side effects
  • Rifampin (e.g., Rifadin)—Use with phenytoin may decrease the effects of phenytoin; your doctor may need to adjust your dosage
  • Streptozocin (e.g., Zanosar)—Phenytoin may decrease the effects of streptozocin; therefore, these medicines should not be used together
  • Sucralfate (e.g., Carafate)—Use of sucralfate may decrease the absorption of hydantoin anticonvulsants
  • Theophylline (e.g., Theo-Dur)—Hydantoin anticonvulsants may make this medicine less effective
  • Valproic acid (e.g., Depakene, Depakote)—Use with phenytoin, and possibly other hydantoin anticonvulsants, may increase seizure frequency and increase the risk of serious side effects affecting the liver, especially in infants

Other medical problems—The presence of other medical problems may affect the use of hydantoin anticonvulsants. Make sure you tell your doctor if you have any other medical problems, especially:

  • Alcohol abuse—Blood levels of phenytoin may be decreased, decreasing its effects
  • Blood disease—Risk of serious infections rarely may be increased by hydantoin anticonvulsants
  • Type 2 diabetes mellitus or
  • Porphyria or
  • Systemic lupus erythematosus—Hydantoin anticonvulsants may make the condition worse
  • Fever above 101 °F for longer than 24 hours—Blood levels of hydantoin anticonvulsants may be decreased, decreasing the medicine"s effects
  • Heart disease—Administration of phenytoin by injection may change the rhythm of the heart
  • Kidney disease or
  • Liver disease—Blood levels of hydantoin anticonvulsants may be increased, leading to an increase in serious side effects
  • Thyroid disease—Blood levels of thyroid hormones may be decreased

Proper Use of This Medicine

For patients taking the liquid form of this medicine:

  • Shake the bottle well before using.
  • Use a specially marked measuring spoon, a plastic syringe, or a small measuring cup to measure each dose accurately. The average household teaspoon may not hold the right amount of liquid.

For patients taking the chewable tablet form of this medicine:

  • Tablets may be chewed or crushed before they are swallowed, or may be swallowed whole.

For patients taking the capsule form of this medicine:

  • Swallow the capsule whole.

If this medicine upsets your stomach, take it with food, unless otherwise directed by your doctor. The medicine should always be taken at the same time in relation to meals to make sure that it is absorbed in the same way.

To control your medical problem, take this medicine every day exactly as ordered by your doctor. Do not take more or less of it than your doctor ordered. To help you remember to take the medicine at the correct times, try to get into the habit of taking it at the same time each day.

Dosing—The dose of hydantoin anticonvulsants will be different for different patients. Follow your doctor"s orders or the directions on the label . The following information includes only the average doses of ethotoin, fosphenytoin, mephenytoin, and phenytoin. If your dose is different, do not change it unless your doctor tells you to do so.

The number of capsules or tablets or teaspoonfuls of suspension that you take or the number of injections you receive depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using a hydantoin anticonvulsant .

  • For ethotoin
  • For oral dosage form (tablets):
    • As an anticonvulsant:
      • Adults and teenagers—To start, 125 to 250 milligrams (mg) four to six times a day. Your doctor may increase your dose gradually over several days if needed. However, the dose is usually not more than 3000 mg a day.
      • Children—To start, up to 750 mg a day, based on the age and weight of the child. The doctor may increase the dose gradually if needed.
  • For fosphenytoin
  • For injection dosage form:
    • As an anticonvulsant:
      • Adults and children—Dose is based on the illness being treated, and the body weight or size of the patient. The medicine is injected into a vein or muscle.
  • For mephenytoin
  • For oral dosage form (tablets):
    • As an anticonvulsant:
      • Adults and teenagers—To start, 50 to 100 milligrams (mg) once a day. Your doctor may increase your dose by 50 to 100 mg a day at weekly intervals if needed. However, the dose is usually not more than 1200 mg a day.
      • Children—To start, 25 to 50 mg once a day. The doctor may increase the dose by 25 to 50 mg a day at weekly intervals if needed. However, the dose is usually not more than 400 mg a day.
  • For phenytoin
  • For oral dosage forms (capsules, chewable tablets, or suspension):
    • As an anticonvulsant:
      • Adults and teenagers—To start, 100 to 125 milligrams (mg) three times a day. Your doctor may adjust your dose at intervals of seven to ten days if needed.
      • Children—Dose is based on body weight or body surface area. The usual dose is 5 mg of phenytoin per kilogram (kg) (2.3 mg per pound) of body weight to start. The doctor may adjust the dose if needed.
      • Older adults—Dose is based on body weight. The usual dose is 3 mg per kg (1.4 mg per pound) of body weight. The doctor may need to adjust the dose based on your response to the medicine.
  • For injection dosage form:
    • As an anticonvulsant:
      • Adults and children—Dose is based on the illness being treated, and the body weight or size of the patient. The medicine is usually injected into a vein.

Missed dose—If you miss a dose of this medicine and your dosing schedule is:

  • One dose a day—Take the missed dose as soon as possible. However, if you do not remember the missed dose until the next day, skip it and go back to your regular dosing schedule. Do not double doses.
  • More than one dose a day—Take the missed dose as soon as possible. However, if it is within 4 hours of your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

If you miss doses for 2 or more days in a row, check with your doctor.

Storage—To store this medicine:

  • Keep out of the reach of children.
  • Store away from heat and direct light.
  • Do not store in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
  • Keep the liquid form of this medicine from freezing. Do not refrigerate.
  • Do not keep outdated medicine or medicine no longer needed. Be sure any discarded medicine is out of the reach of children.

Precautions While Using This Medicine

Your doctor should check your progress at regular visits , especially during the first few months of treatment with this medicine. During this time the amount of medicine you are taking may have to be changed often to meet your individual needs.

Do not start or stop taking any other medicine without your doctor"s advice. Other medicines may affect the way this medicine works .

This medicine will add to the effects of alcohol and other CNS depressants (medicines that may make you drowsy or less alert). Some examples of CNS depressants are antihistamines or medicine for hay fever, other allergies, or colds; sedatives, tranquilizers, or sleeping medicine; prescription pain medicine or narcotics; barbiturates; other medicine for seizures; muscle relaxants; or anesthetics, including some dental anesthetics. Check with your doctor before taking any of the above while you are using this medicine .

Do not take this medicine within 2 to 3 hours of taking antacids or medicine for diarrhea. Taking these medicines and hydantoin anticonvulsants too close together may make the hydantoins less effective.

Do not change brands or dosage forms of phenytoin without first checking with your doctor. Different products may not work the same way. If you refill your medicine and it looks different, check with your pharmacist.

If you have been taking this medicine regularly for several weeks or more, do not suddenly stop taking it . Your doctor may want you to reduce gradually the amount you are taking before stopping completely.

Your doctor may want you to carry a medical identification card or bracelet stating that you are taking this medicine.

For diabetic patients:

  • This medicine may affect blood sugar levels. If you notice a change in the results of your blood or urine sugar tests or if you have any questions, check with your doctor.

Before you have any medical tests, tell the doctor in charge that you are taking this medicine. The results of some tests (including the dexamethasone, metyrapone, or Schilling tests, and certain thyroid function tests) may be affected by this medicine.

Before having any kind of surgery, dental treatment, or emergency treatment, tell the medical doctor or dentist in charge that you are taking this medicine . Taking hydantoin anticonvulsants together with medicines that are used during surgery or dental or emergency treatments may cause increased side effects.

This medicine may cause some people to become dizzy, lightheaded, drowsy, or less alert than they are normally. After you have taken this medicine for a while, this effect may not be so bothersome. However, make sure you know how you react to this medicine before you drive, use machines, or do anything else that could be dangerous if you are dizzy or are not alert .

Oral contraceptives (birth control pills) containing estrogen or progestin, contraceptive progestin injections (e.g., Depo-Provera), and implant contraceptive forms of progestin (e.g., Norplant) may not work properly if you take them while you are taking hydantoin anticonvulsants. Unplanned pregnancies may occur. You should use a different or additional means of birth control while you are taking hydantoin anticonvulsants . If you have any questions about this, check with your health care professional.

For patients taking phenytoin or mephenytoin :

  • In some patients (usually younger patients), tenderness, swelling, or bleeding of the gums (gingival hyperplasia) may appear soon after phenytoin or mephenytoin treatment is started. To help prevent this, brush and floss your teeth carefully and regularly and massage your gums. Also, see your dentist every 3 months to have your teeth cleaned. If you have any questions about how to take care of your teeth and gums, or if you notice any tenderness, swelling, or bleeding of your gums, check with your doctor or dentist .

Side Effects of This Medicine

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor as soon as possible if any of the following side effects or signs of overdose occur:

More common

Bleeding, tender, or enlarged gums (rare with ethotoin); burning, tingling, pain, or itching, especially in the groin—following fosphenytoin injection; clumsiness or unsteadiness; confusion; continuous, uncontrolled back-and-forth and/or rolling eye movements—may be sign of overdose; swollen glands in neck or underarms; fever; muscle pain; skin rash or itching; slurred speech or stuttering—may be sign of overdose; sore throat; trembling—may be sign of overdose; unusual excitement, nervousness, or irritability


Bone malformations; burning pain at place of injection; chest discomfort; chills and fever; dark urine; dizziness; frequent breaking of bones; headache; joint pain; learning difficulties—in children taking high doses for a long time; light gray-colored stools; loss of appetite; nausea or vomiting; pain of penis on erection; restlessness or agitation; slowed growth; stomach pain (severe); troubled or quick, shallow breathing; uncontrolled jerking or twisting movements of hands, arms, or legs; uncontrolled movements of lips, tongue, or cheeks; unusual bleeding (such as nosebleeds) or bruising; unusual tiredness or weakness; weight loss (unusual); yellow eyes or skin

Rare (with long-term use of phenytoin)

Numbness, tingling, or pain in hands or feet

Symptoms of overdose

Blurred or double vision; clumsiness or unsteadiness (severe); confusion (severe); dizziness or drowsiness (severe); seizures; staggering walk; stuttering or slurred speech

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

More common

Constipation; dizziness (mild); drowsiness (mild)

Less common

Diarrhea (with ethotoin); enlargement of jaw; muscle twitching; swelling of breasts—in males; thickening of lips; trouble in sleeping; unusual and excessive hair growth on body and face (more common with phenytoin); widening of nose tip

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.

Additional Information

Once a medicine has been approved for marketing for a certain use, experience may show that it is also useful for other medical problems. Although these uses are not included in product labeling, phenytoin is used in certain patients with the following medical conditions:

  • Cardiac arrhythmias (changes in your heart rhythm) caused by digitalis medicine
  • Myotonia congenita or
  • Myotonic muscular dystrophy or
  • Neuromyotonia (certain muscle disorders)
  • Paroxysmal choreoathetosis (certain movement disorders)
  • Tricyclic antidepressant poisoning
  • Trigeminal neuralgia (tic douloureux)

Other than the above information, there is no additional information relating to proper use, precautions, or side effects for these uses.

Revised: 05/01/2000

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Typical mistypes for Cerebyx
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